I agree with the general thrust of your point, but I'm not sure it applies here. People smoke weed because it's fun or because it is medically useful. But from the tobacco smokers and ex-smokers I know, that's never the case. It's not fun; it's a very hard-to-kick addiction.

It's also an addiction provided on a for-profit basis. That means strong incentives to create new addicts. I don't think it's at all unreasonable for us as a society to work against that.