That's because lying is a way of life for these vermin. Just like in this country where lying is not cause for impeachment if you are a member of a particular political party.
Note to self: America and American properties only.
Janssen (the Aruban attorney general) said the elder van der Sloot gave his son and the Kalpoe brothers legal advice, telling them that "without a body there is no case."
Just thinking out loud, but this must occurred before the boys were picked up originally (he hasn't had access since that time). And the fact that PVDS told the boys this raises the question of why would he say that unless he (1) knew they were guilty or had some involvement in "something bad" and (2) he knew that body wouldn't be found.
At that early date, even he might have thought she had just run away or drowned while swimming at the beach...that is,unless he knew for a fact that the body would not be found or had first or second hand knowledge of what was done with the body.
Bottom line...what made him think the body wouldn't be found? The girl might have been just missing...like the runaway bride.