Originally Posted by punkworld

Two reasons for that. The first is tradition - lawbooks aren't written on a tabula rasa, but evolve through continuous tinkering with existing customs and laws. The second is that while smoking and drinking generally don't prohibit one from leading a mostly socially acceptable life, most "hard" drugs do. Of course, laws also play a large role in that (by driving up prices and thus necessitating crime for most addicts), but the general belief is that while a smoker who drinks a few glasses a night will probably still be able to function in society, a heroine, crack, meth or coke addict will not. Experiments with free drugs for addicts are partially undermining that belief at present, but it still remains strong in most people's minds, and also seems to have at least some roots in reality.