Outlaw crimes, not plants.
People who are baked or half baked on the road, driving a semi truck, operating a crane, teaching children, performing surgical operations, or otherwise endangering the public by being partially incapacitated have committed a crime. That's why drugs are illegal.
No, my little liberal wannabe tyrant, crimes are not committed until they are actually committed. This notion that just because something is legal that everybody and their brother will automagically go stupid and start doing everything impaired is silly and not supported by reality. Indeed, using your logic, everybody who uses alcohol MUST be drunk "on the road, driving a semi truck, operating a crane, teaching children, performing surgical operations, or otherwise endangering the public by being partially incapacitated"? Are they? Are you?
Try again, this time try thinking with your brain instead of your dipstick, jimmy.