A thought experiment I think about is along the lines of: what would society look like, say, 10,000 years in the future, if everybody somehow magically had an Einstein-level of intelligence and rationality. In such a society, sure, the government probably wouldn't need to step in; the vast, vast majority of the population would either have little interest in the drugs in the first place, or, if they did, could be trusted to partake responsibly.
However, that's not the world we live in. We share our cities with fairly unintelligent, irrational people, that have no interest in higher ideals. Our cities are being destroyed and made unsafe by these people that are just out of their minds on drugs / mental issues, completely disconnected from society, vandalizing, breaking and entering, hurting other people. They obviously, demonstrably, can't be trusted to partake responsibly.
I guess the debate is to what level the government needs to step in to control such people and the actions they take. I'd say that since they've already demonstrated they can't be trusted to coexist with peaceful society, that some level of action needs to be taken. But it's tough because in an ideal society I'd say the correct thing is for the government to stay out of it. But we live in a far from ideal society.