Is the prohibition of cannabis justified?

I don't think so... I don't think any of the drug laws are justified. Congress (correctly) decided that it was unconstitutional to p any drug laws for most of our nation's early history, which is why they decided to make it a "tax act" to criminalize marijuana, a sneaky and subversive move in itself, and then eventually just forgot about that and decided to go ahead and ignore the unconstitutionality of the laws. And yes, as your links point out, weed does far less harm to America than alcohol already does. (I'm not a marijuana user, either, just objective)