My thoughts are just the opposite. I can't see paying $150 for an "arm brace" when an SBR stamp is $200.
The arm brace is a poor substitute for a real shoulder stock.
Let's not forget that that are a bunch of military folks who are moving state-to-state every couple of years, without much choice of which states they're being moved to.
That makes it nearly impossible for those people to own NFA firearms.
The brace suddenly now allows those same people to own SBR-like firearms.