Sounds like one is "damned if they do, damned if they don't" (follow what the rental agency requires). I agree with @wwguy's speculation. Avis contracts all over the world might have that same boilerplate "secured" language in the fine print, and poorly trained but well-meaning employees might open their mouths about it. If this is indeed the case, then if you obey the contract you risk the hassle of a broken window and five bucks for new sunglasses. But if you use your judgment and do what everyone on Bonaire does, then you violate Avis' terms (again, assuming the speculation about the contract fine print is correct), and there might conceivably be some circumstance under which you end up on the defensive. (I can't think of any, since no one in their right mind leaves valuables in a Bonaire rental vehicle, but maybe there is such a circumstance.)
How about theft of the vehicle its-self? Would you be liable if one of the thieves stole an unlocked vehicle?