If cannabis use results in higher health costs.

However, the preponderance of research suggests a null hypothesis at worst and a reduction in cancer rates and opiate dependency at best.

I’m honestly curious how many people don’t do drugs solely because it’s illegal? How many people are thinking “I’d love to try meth; if only it were legal!”

If you're going to reason about the effects of this the collective society you must also consider the cost/benefits of continued prohibition to the society as well. We are already paying for the decisions of the state to imprison people under prohibition. Is this cost more or less than the cost incurred to national healthcare of individuals poor health decisions?

Fair point, but you could say the same thing about skiing or any other potentially risky human activity. The point of society isn't to minimize health care costs but to maximize happiness.

In principle, it’s mostly other consumers of canabis who will pay, via a “sin tax”. That’s already the case with cigarettes and alcohol.