Woady : Which you will need to SBR and get a suppressor for
RRaisley, looking for logic in a government organization or rule is always a good way to waste time. There doesn't have to be any logic.
In the USA it is illegal to put a stock on a pistol without first registering the lower as a SBR - Short Barreled Rifle. If you do, it is I believe, a felony. Something like the Roni kit for a Glock, or even the Glock shoulder stock is a no no. Why? Who knows. The shoulder stock'd Glock is hardly any more dangerous than the normal Glock, and harder to conceal, but there it is.
If you do SBR a pistol, something that can be done legally with a Form 4 (IIRC?) you can not return it to the original pistol state. It is now a registered rifle. Making a rifle out of a pistol can ease some burdens of trying to make a longer style gas system work in a shorter space. Something like the DRAKO AK pistol makes a very fine SBR AK rifle, as the short pistol length gas tube works better than trying to shorten the longer rifle length gas system. There may be a touch more gunsmithing to fit a stock to a pistol, but the paperwork is the same, IIRC.
Take what I say with a pinch of salt, I do not live in the USA.