It's also clear that young people shouldn't smoke pot. It's bad for athletic and academic performance, and it can exacerbate emotional problems, too. So can other substances, which are legal. I think it has everything to do with who those users are. This society does not scorn all drugs.
Alcohol is very respectable.
We even allow beer ads on MTV, a network aimed at people 12-24 years old. But pot is different. The war on marijuana has little to do with health. It has everything to do with culture. It's a moral crusade.