"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.
The blood runs free, the rain turns red, give me the wine, you keep the bread.