They are imported. Can't have a threaded muzzle on an imported firearm. That's the 89 import ban at work.
ATF has determined that certain features designed for military application are indicative of non-sporting rifles and shotguns. Features which are not recognized as sporting include, but are not limited to, folding or telescoping stocks, pistol grips that protrude conspicuously beneath the action of the weapon, a bayonet or bayonet mount, a flash suppressor or threaded barrel designed to accommodate a flash suppressor, a grenade launcher and night sights.
Leaving cold: https://www.atf.gov/firearms/firearms-guides-importation-verification-firearms-ammunition-and-implements-war-import
ETA - I'm rolling eyes at the ATF, not you, to clarify.
There are FAL threads under that muzzle device if you drill out the pin.