...seeing as I don't want to push the nanny filter at work more then I usually do.
Part of the appeal to these is that by completing that last 20% yourself, you don't have to put a serial # on the firearm. However, you can't sell it to anyone, either. (It's still legal for a citizen to make firearms in the US for their own personal consumption; however, the vast majority of folks don't have a CNC mill or a machine shop handy to fabricate an AR from a billet.)
IIRC, the legal trouble was that the shop had gotten a little too close to that edge where the ATF says that it's a person doing the work, and the shop doing the work. Again, I'll dig through my sources tonight and see what I can find.