I'd argue that this is because of what seems like a mismatch between local / state laws and federal laws.
I'm not from the US, so don't know how all this works, but I assume that if weed is "generally legalized", then it should be at the federal level, too.
Our constitution is supposed to relegate intrastate trades to the states via the 10th amendment.
This was nullified in wickard v filburn, where they decided growing your own plant and consuming it on the spot is interstate commerce. Obviously this is nonsensical, but the courts don't want to shake the house of cards and you'd have people screaming about the "end of the civil rights acts" and various other popular laws that depend on an extremely fudged version of interstate commerce and the 10th.
I'm not from the US, so don't know how all this works, but I assume that if weed is "generally legalized", then it should be at the federal level, too.