I think DOJ is combining the figures for 'serial removed' firearms and 'not serialized' guns.
Personally I think a good way to solve this is to require background checks for kits and require anyone manufacturing a firearm to serialize said firearm (listing your name, city, and state on the frame as any other OEM would) and register said firearm if your state requires such for possession. (Not that I want them to require it!) That way it is very slightly harder for those without the ability to possess in a given state to acquire these kits, but anyone who is able to possess legally is not affected. This would of course move the industry from advertising them as 80% firearm kits to 'model/airsoft' gun kits or something similar, but such is how legal matters go....
Similar rules apply for 'experimental' automobiles and aircraft, so I don't see a difference here.