John Richardson points to a copy of an ATF ruling on Airsoft replicas, which has since disappeared. The end result may be kind of absurd, but the fact is, if you can pop out the Airsoft bits, pop in an AR-15 or M16 trigger group, throw on a real AR-15 upper and bolt assembly, and get the thing to go bang, the Airsoft receiver is by law a firearm, and if it goes full auto, then the receiver is also a machinegun. The law regulates firearms or receivers of firearms. An AR-15 lower does not undergo all that much mechanical stress. Most of that is in the upper receiver. It would be perfectly safe to fire an AR-15 with a pot metal lower receiver. Might not stand up to abuse, but you’ll be able to use it.
So I don’t think this is really something ATF deserved to be mocked over. It sounds as if some of these Airsoft replicas are indeed close enough to AR-15/M16 receivers to function as such. The law might be absurd, but that’s Congress’ fault, not ATFs.