In some states, it is, sort of.
Affirmative defense I believe the term is, but somebody can correct me.
In some/lots of states, NFA is technically illegal, HOWEVER if you comply with all federal laws its legal. Your stamp or copy of stamp is your "affirmative defense" that you are complying with the federal law. So in essence, in these states, your stamp is your permission slip and proves you are legal.

