You are right, and the BATF whistleblower website has ATF agents basically saying the same thing. It is splitting hairs over whether or not the airsoft lower is equal to or less than an 80% unit, as the shims and custom parts put completion in a different category than the machining necessary on a blank. What most people agree on in every forum of well informed people is that converting one of the WE-Tech M4s to live fire is not cost effective, even on the black market in the US.

Interestingly in checking the net, all of the WE tech gas blowback airsoft guns are being bought up now in droves, even the models not particularly targeted by the feds.

You get the impression from that other agent's commentary about Crenshaw and the airsoft issue that they do want to go after the airsoft guns, but with a multi-agency approach which says letting your kid run around with something that looks like a real weapon is child endangerment, therefore sidestepping the whole technical issue anyway.