I don't know what BATFE has decreed. I am sure they haven't limited themselves from regulating it because they want to be nice. They haven't regulated it because the can't stretch the commerce clause to a gun that is never going to be bought, sold, or traded. That is it. They have restricted as far as they think they can without pushing it to where they have to worry about a court case that takes away some of their power.
See Wickard v. Filburn, which is still law of the land. Roscoe Filburn had a bunch of wheat that he was never going to sell or trade - just consume on his farm. And according to SCOTUS, the commerce clause extended to the feebs the right to regulate/tax the wheat and/or prohibit him from growing it. There are precious few transactions which a jurist with a generous view of the commerce clause cannot squeeze into the tent.