"But the guy who used the marijuana will still have the federal law problem of being a prohibited person in possession of a gun. That can be good for up to five years in the federal slammer and comes with a lifetime loss of gun rights."
Only because the federal law is out of step with contemporary society.
The last time I filled out a 4473 I thought it was the most ridiculous thing I have seen. In the opinion of the gov't if you are a user of marijuana you are a menace to society, however you could be a raging alcoholic (a TRUE drug addict) and it is still ok to fill out the 4473 and get your gun.
Give me metal or give me death \m/