The medical marijuana provisions are so loose in formulation and enforcement as to be a ruse for recreational marijuana. In addition, the grow industry cannot be separated as to marijuana intended for instate medical purposes and commerce to other states.But just because there is a potential problem there, does it therefore mean the federal government should just assume powers not granted to it by the Constitution? Seems to me that is the line of thought that gave us the behemoth fedgov we have today - because there were problems that needed solving and the feds just usurped the powers to try to solve them (emphasis on the word 'try').
The expansion of the federal government was not founded on opposition to medical marijuana. And the problems that I refer to are not mere "potential problems" but current and substantial problems.