Ray - Probably because like with the 7.62 x 39, to have a short-barrel model that was legal, some idiot company or gunsmith made a (some) firearm(s) in pistol form - that is, short barrel, no buttstock - to sell to the Walter Mitty part of the shooting public. If a licensed manufacturer does that, it does not come under the law of “manufacturing a pistol from a rifle,” which is part or all of the thing that prohibits cutting the butt part of the stock off, and shortening the barrel way down. It is a matter of licensing. It is illegal for a private, unlicensed person to do that, but if a company with a manufacturer’s license makes a new gun that way and designates it as a pistol, then it is legal under Federal law. Just a sample of the idiocy of gun laws in general. My disdain for that type of firearm is because they are basically good for nothing in rifle calibers, satisfying only the urge to have a SMG to play with, unpleasant to fire, and too large to be of any real use for civilian self-defense.(CCW). The only thing sillier than those rifle-caliber SMG look-alikes is the half-law that prohibits them if not made or converted by a licensed manufacturer, and the laws that cause normal rifle ammunition to all of a sudden be declared a pistol cartridge because of a few hybrid guns (possible a single specimen, even). Laws that prohibit guns on the basis of barrel length, so-called “assault weapons” features like flash hiders, bayonet lugs, or pistol grips, or simply because of their color or what they look like, are childish at best, the product of lawmakers with diminished intellect and a psychotic fear of firearms. I would not attempt to describe my feelings towards those that pass laws that declare rifle cartridges to be pistol ammunition because of a few guns in that caliber. I would have to enter the realm of obscene language.
