This was chatted about when it happened.
The ATF theory is that you can take the GBBR lower and mix it with an AR upper (which are as common as dogshit in the USA) and presto instant machine gun. It did take a few false starts for them to articulate their logic.
Never mind just how many real deal weapons can be found in the US black market, same logic as is used in Canada. Go after folks who aren't breaking the law or murdering people as they're just plain nicer to deal with.
Airsoft, where nothing is hurt but feelings.