Just the screwed up way the BATFE works. AR's are the only bare receiver I can think of that MIGHT come with a stock from the factory.
Like I said it is still a long gun in the eyes of the BATFE. If for some reason they went to the manf. and saw that it left the factory with a stock and is in pistol form now you better have a form 1 or form 4 to go with it.