Winston -
The fact that Cash had an AR15 was completely irrelevant to the decision. The court determined that although BATF had the authority to determine whether a DIAS was a machinegun or not, BATF did not have the authority to exempt some DIAS's from registration based upon their date of manufacture. From the decision:
[b]But nothing in the firearms statutes gives the Secretary
of the Treasury (or the Bureau of Alcohol, Tobacco and Firearms)
the power to make exemptions to section 5845(b) and associated
legal obligations. The statute to which ATF Ruling 81-4 refers, 26
U.S.C. section 7805(b), provides that the Secretary cannot give
retroactive application to tax regulations and adds in section
7805(b)(8) that the "Secretary may prescribe the extent, if any, to
which any ruling (including any judicial decision or any
administrative determination other than by regulation) relating to
the internal revenue laws shall be applied without retroactive
effect."[/b]
Thus, according to the court, a DIAS is in itself a machinegun. It is either registered or not, and if not registered, its contraband. The 1981 date is relevant only for purposes of a tax free registration; if you had a pre-81 sear you could have registered it without paying the $200 making tax if you did so prior to May 1986 when all civilian registrations of new MGs was cut off bu 922(o).
I suspect one reason BATF has not pushed this as hard as they could in other jurisdictions is that to some degree Cash was a loss for BATF too. Their own rule regarding DIAS's got shot down by the federal court in favor of a more strict rule. It not only makes more work for BATF, especially when you consider the implications with semiauto open bolt guns, but they also lost face and got scolded by the court. The other reason is either (a)that BATF feels sorry for the guys who have stockpiled unregistered DIAS's and sell them in SGN and are just being really nice and looking the other way while they give them a chance to unload their inventory, or (b)they're compiling a list to track down later (now which do you think is more likely?).