I think it's because weed users refuse to admit any harm could come of what they do. Alcoholics and other drug addicts know full well they are doing something very destructive. Most potheads openly talk about it in a 'and this isn't doing me any harm so it's fine to talk about' way.

Unfortunately, it does do harm, as anyone who has seen a friend become a pothead knows, although the harm is not nearly as obviously destructive as many other drugs.