Prohibition in general creates a lot of problems, but I'm not sure littering is one of those. Even with items that are not prohibited—as you mentioned in the title: water bottles, gatorade bottles—people litter. Smoking isn't illegal (yet), but people litter their cigarette butts everywhere. Here in Japan it isn't illegal to carry around trash, yet people litter it everywhere. I pick up trash some mornings and I know others who do it every morning and we always end up with at least one trash bag full, often more. People litter because they are lazy and because they don't think they are doing any great harm (or they convince themselves of this). Now in the case of beer cans, not wanting to be caught with them in the car certainly doesn't help, but it is far from the cause.