“This is to ask, what in the Constitution allows the federal government to do so?”
Don’t hold your breath. I’ve asked FReepers (who advocate the federal drugs laws) that question countless times, and I think only one actually bothered to try to craft a response.
I know — It's why I ask.
It essentially invites them to defend the indefensible: support of the War on Drugs is essentially, at its core, rejection of the constraints of the Constitution.