bit of a sad story... a friend of mine's little cousin is the reason the no kids in the bed law was passed in FL...
drunk daddy lets 3 y/o (or so) girl sit in the bed of the truck IN her new plastic pool (you know the little kiddie pools)... for whatever reason a gust of wind picked up the pool and the girl inside and well... the rest of the story is kinda obvious :-/....
oddly not all states have laws against riding in the bed of a truck...
that being said... as a kid I loved riding in the bed of trucks... and rarely did they have a cap (the law in FL lets you ride back there if it's enclosed as long as its not limited access road)