The law (ITAR[1] and the Arms Control Export Act[2]) wasn't designed to keep home-milled ArmaLite parts out of the hands of US citizens. The goal was to prevent someone like Lockheed from doing something like selling stealth fighters to an adversary without checking in with someone first. It's basically written as banning the export of military technology to foreign countries without the proper paperwork.
Apparently, to the folks at the State Department, stuff like PGP[3] and plans for a 3D-printable gun count as "military technology" and putting it on the internet counts as "export". That's pretty different from the ATF deciding that a lower receiver without holes in it doesn't need a firearms dealer for a transfer.
[1]http://en.wikipedia.org/wiki/Arms_Export_Control_Act
[2]http://en.wikipedia.org/wiki/International_Traffic_in_Arms_R...
[3] http://stason.org/TULARC/security/pgp/99-Appendix-VI-A-State...