It's interesting that even your quick summary of the law uses the phrase "when building." That's the way the law is written. I've argued before that it applies to the person who builds it but that's all it says.
So if Fred Flintstone down the road builds a non-compliant FAL, then sells it to a gun store, and then I buy it and bring it home... I can't see how I am the person subject to the law. I didn't build it. The receiver was purchased by somebody else. Etc.
I just find it hard to believe that somebody could be "guilty of breaking the law" when all they did was buy what they thought was a legal product. A product that was represented as being legal. Which appears to all general inspection to be legal. Maybe the guy who built it knew better. Maybe not. But how does that apply to the 2nd, 3rd, or 4th person down the chain? I can see a "good cop/bad cop" situation where it is used a threat to get cooperation but it is really, really hard for me to see how it gets past a judge in a courtroom....