Unnecessary complication
What has always been troubling is not that Joran van der Sloot and the Kalpoe brothers first lied about dropping Natalee Holloway off at the Holiday Inn. It is reasonable to understand anyone wanting to avoid "embarrassment" after having done something uncouth.  Leaving a girl alone on the beach in the night is ungentlemanly by most standards and not something a young man would want family and friends to know about.  Aruba is a vacation paradise, and beaches are frequented at all hours of the night.  So, it might be logical to assume that a girl left alone, even in the wee hours of the morning, would have been just fine, might have met up with someone else and eventually would show up.  Therefore, a reasonable person might assume that no harm would come from providing a simple, but more "respectable," explanation, that she was taken to her hotel.  Girls on vacation often connect with others on the island and often spend the night in unplanned places. So, it is not a stretch of credibility that the girl could have been expected to show up later on her own.  Under ordinary circumstances such a cover story would be have been a harmless tale. 

But, what is bothersome is that this "cover" story, this lie, was too complicated.  It is too detailed, too convoluted to be excused as just an off-hand explanation given to avoid embarassment.  They all told a lie with so many unnecessary details.  We heard that she fell upon getting out of the car, that Joran went around to help and that Natalee said she didn't want assistance. Then, they added the security guard scene, complete with a description of the clothing. It is all so unnecessarily compounded. If there had been a simple story, such as, "We dropped her at the Holiday Inn and waved good-bye," that would fit the format of an "I-don't-want-to-look-bad" excuse.  Once we find that this "story" was a lie, however, the question immediately becomes, "Why was the lie so complicated?" Experienced interrogators know that such a complicated lie often provides valid reasons for suspicion of something hidden.  A complex lie, when a simple one would have sufficed, is often an indication that the prevaricator is wanting to convey information to explain away circumstances which might come out later, such as an injury subsequently discovered on the body of a victim.  "She fell when getting out" might have been offered to provide a reason for an head contusion or cut, which might have been anticipated as being discovered later in an autopsy.

Failure to correct quickly
Irrespective of a cover story being logically excusable and told "just out of embarrassment," when Natalee didn't show up after a day, anyone with scruples would have told then what really happened.  A, "keeping up my reputation" excuse,  with a rapid correction after a day passed, might be explained as "17 year old" thinking.  The key for forgiveness is "instant" correction.   Doing the right thing cleanses.  But, the grace period would be a matter of hours.  Don't forget, all three knew they were preserving a blatant lie, while people were desperate to find her, while they knew police were investigating.  Anyone would know; time is critical in finding a missing person. It is obvious that the three intentionally and knowingly misdirected the police in the most crucial point of the investigation. Plus, they misdirected for a very long time; at least nine days. Why did they do that? That remains the burning question.

It is reasonable to expect a moral compass
So, applying reasonable standards, most would expect them to have had the proper "moral compass."   It would take a certain, determined callousness not to fix the falsehood with the truth, when things looked worse than they originally thought.  Certainly, after 24 hours, someone who knew that her family had been to Joran's home would feel an obligation to want to help find her.  What motivation would have kept anyone from telling the police, as well as everyone looking, that the original story was completely not the truth?  It is reasonable to think that, of the three, at least one would have wanted that search to have begun in earnest from the last location where she was seen. 

So,  the lie wasn't told simply and, most important, the lie wasn't abandoned quickly.  It was only after ten days had passed and they were arrested and shown the surveillance video tapes from the Holiday Inn.  You have to ask, "Why?"

For at least nine days the three intentionally misdirected the police.  Why?

Perhaps they knew a search would be futile
Why would someone not feel the urgency to reveal the truth about where they left a missing girl?  If you left her on a beach and she hadn't shown up after a day, wouldn't you want to get  teams searching from that location?   Perhaps, they didn't because they knew it was futile.   We know for certain that none of the three showed any true desire to help look for Natalee. They would have corrected their deception. They would have helped in the search. Instead of helping to findher,  they repeatedly (by Joran's admission) met at Joran's house to firm up their "story." Was it because they knew full well that there was no emergency?  A reasonable explanation is that they all knew she was already dead.  Maybe they knew that her body was not even on the island.  It is hard to explain any other way: three people not feeling any compulsion or compassion to want to help in her search.  Where was she was last seen? What did you see in the area? What are other details that needed to be told?  Once they knew she had been gone way too long to have just "slept in," unless they had reasons to know a search would be in vain, they would have been out there, showing right where they left her and then helping in the search.
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Perhaps they needed time to cover a crime
Coupled with knowing there was no urgency, not coming forward quickly, fits a pattern of criminal behavior where activivites are done to delay, to allow a crime scene to be cleaned, a body disposed, evidence to be destroyed.

Perhaps they wanted even more time 
They secured more time by allowing the Security Guards to remain in jail. That is undisputed. This sequence is probative that they were securing time before revealing the truth (or what they say is the truth). Was it to cover their tracks?   It is problematic because it fits into the "modus operendi" of a criminal "buying time," delaying for his own self interests.  Allthough they didn't specifically frame the guards, they didn't jump to their defense when those two were arrested. Common respect for one's fellow man, being falsely accused, should have motivated them to instantly tell their beach story (assuming even that's the truth) abandoning the Holiday Inn prevarication. What reason did they have to prolong that situation?  Why did they need more time?

Four concerns which warrant suspicion of wrong doing
1. Unnecessary complication (for someone just telling a fib to spare embarrassment). 
2. Not correcting the story when it was apparent she wasn't coming back after a full day.
3. Not correcting the story the very moment innocent people were arrested. 
4. Prolonging their revealing of the last place they saw her for up to ten days.

Something bad evidently did happen
So, because of these concerns it seems apparent to many that something isn't right. It appears that something bad did happen. These logical steps cause a reasonable person to now question the beach story, giving rise to speculation that they chose that place for the second story as just a more coverable lie.  There were no cameras there to refute it (like the Holiday Inn) and simply it seems to be more conceivable than the first. But, if the second story is also a lie, where did they really leave her? 

In spite of these concerns, most people do remain believers in "IUPG" (innocent until proven guilty) and in needing evidence which convinces beyond a reasonable doubt. Those very principles may mean that this crime is never solved. It is a basic tenet of our free society that no one (even the guilty) should ever be railroaded into a conviction based on hunches and gut feelings. So, we must direct our efforts toward attempting to sort out facts from theory and rumor. 

However, it is difficult not to have justifiably strong suspicions of guilt.  Certainly, all the circumstances of the admitted lie make such suspicions credible. What the authorities need is some evidence. Hopefully, this same position is shared by the Aruban Police, the Prosecutor and the FBI. 

There is yet another problem with the two stories, which is realized with a simple bit of legal deductive reasoning. While the following discussion may be an exercise in legalistic logic it can serve to gain a very interesting perspective on why so many people remain convinced (in spite of no evidence of a crime) that the three suspects are guilty.

US case law which speaks to this situation:

Both stories are exculpatory
Saying the two versions are both exculpatory means that both of them taken at face value (by themselves) are stories which describe no criminal involvement. They serve to clear the suspect of any wrong doing. 

In a criminal proceeding, if a witness tells one accounting and then recants, and then tells what he now says is the truth (usually in the face of some evidence from another source, which would be incriminatory), you look for the reason he told the first lie. Invariably, the sequence makes sense, because while the first accounting was exculpatory (describing innocence), the second story (the truth) will be in some way incriminating (non exculpatory), therefore explaining or demonstrating the reason or need for the original lie. 

So the logic to be applied is that when you have two stories, and the first one is recanted for the second, but both are exculpatory, then you also suspect the second, because a recanted exculpatory alibi should logically be replaced with a story which confesses to, at least, some criminal activity (for example, accidental death and hiding a body or something worse). 

Where does this logic take us? It takes us to the understanding that, according to the experiential record of practiced interrogators, Joran's second story is also a lie or, perhaps, a partial truth. Maybe he did leave her on the beach. So, that part would be the truth (making the new story easier to tell and swear to). The various theories could then apply: Maybe she died accidentally. Maybe he went back and buried her (maybe with Deepak's help). Maybe, later he made arrangements (with perhaps a drug acquaintance) to have the body removed and disposed of at sea. Pure speculation, of course. 

It is important to consider that in the practice of studying testimony and applying logic to the way witnesses testify, especially concerning their own alibis, suspects who provide an exculpatory alibi and then recant that story, have traditionally done so because the truth (which they were trying to hide with the first story) describes their involvement in a crime. 

Joran's wanting to avoid embarrassment reason for telling his first (now admitted lie) simply doesn't rise to an appropriate level to assume any kind of logical justification for providing a such a complicated cover-up story.

Further US case law references on this subject:

It is easy to see why so many people (including the television show talking heads) remain convinced that at the least the three suspects have not told the whole story and worse they are covering up for a serious crime which some speculate was disposing of the victims body after an accidental death and some think even murder.  Hopefully, investigators are not at a dead end. The best approach in some people's opinion would be attractive awards for people giving information, which would lead to the arrest and conviction of anyone who participated in a crime. 

Courts in many circuits have approved giving  juries instruction of this nature:
      "False exculpatory statements are properly admissible as substantive evidence tending to show consciousness of guilt."