> I have personally seen enough people that smoked themselves stupid or into poverty

We mostly don't criminalize self-harm. Is it really better for those people to be in prison, with violent criminals, and have a record, than be stupid or poor from too much smoking? Who does that help?

Drunk people get into accidents and fights and even commit murder. Cannabis users mostly stay home and snack. And yet we don't lock up people for buying a six-pack.