Just because cannabis is illegal under federal law doesn't mean that the patients under state medical programs are illegal users. That's the part you're not comprehending.
All we know is that there is a possible conflict between the state laws and federal law, which invokes the supremacy clause, meaning only the supreme court is going to be able to answer the question. But that's never going to happen, because congress has precluded the justice department from ever bringing that case to trial in the first place.
SCOTUS WILL NOT DECIDE THIS. It will take action in congress to legalize MJ recreational or medical before anything changes in the 4473. I see this happening in the future, but front line FFL's cannot speculate, only comply.