Put yourself in their shoes, 3rd...
If you're 18, and they sell you a parts kit that you use to finish out a fully automatic whatever, and turn out to be another Dylan Klebold/ric Harris clone, and take your teen rage out against the nearest local skool, that exposes them to risk beyond what they have the tolerance for.
I don't know the fine gradations of the law, but they are protecting their business interests by reducing their risk.
Sorry. I feel your pain, it's just how it is.