Another possibility here is, some of the ammo released as "surplus" was actually nothing of the sort, and was refused as unacceptable, and ended up in the civilian market, and simply marked for SMG's as a liability disclaimer, so people wouldnt shoot it in pistols.
We got a bad batch of 7.62x51 (if I remember right, it was CBC head stamped) back in the 80's out of SGN, that was recalled soon after, due to guns shooting it blowing up. It was sold as "surplus", but Im not aware of any 7.62x51 being loaded "hot" for issue use, so we always assumed it was a bad lot that was refused and got into the system.
This is what I believe. Recently I found that ammunition has a shelf life. Powder does not deteriorate to a benign condition, instead , at the end of its lifetime, it has burn rate instability issues and pressures climb. Old ammunition has and will continue to blow up firearms.
People have not been educated on this, in fact they have been told the exact reverse, that old ammunition fails safe. It does not.
Surplus ammunition is out on the market because the ammunition technicians responsible for stockpile surveillance determined the stuff was unsafe for their troops to shoot, and unsafe to store. A nasty, which few are aware, is the propensity for old gunpowder to auto combust.
As to whether there is SMG only ammunition, as others have stated, if extra hot SMG only ammunition got into a service pistol that would result in a damaged pistol. It would be difficult to keep the hot stuff out of the hands of troopers who would probably load it in their pistols precisely because they want “hot ammunition” under the premise that hot ammunition would cause more damage to the enemy. The end result would be worn out and blown pistols. I am therefore skeptical, though it could be, a could be, that there was special ammunition made only for SMG’s. You can look at US military 45ACP specs, there is no SMG only specification. The same 230 FMJ with 5.0 grains Bullseye functioned both the M1911 and the Thompson.
I suspect what was sold as “hot SMG” only ammunition was ammunition that was way past a reasonable shelf life and was overpressure due to age. Because people are unaware of the problems with old ammunition, in fact, those sitting on piles of old ammunition deign it if told so, folks accepted the sales pitch.