Good point -- never really thought about that. Many (but not all) 22 LR loads marked "standard velocity" are pretty close to the speed of sound at the muzzle, depending on barrel length.
Possibly the idea is that the loads specifically marked as subsonic are more likely to be too light to cycle a blowback action and will fail to extract. The real answer would have to come from GM themselves.
I went looking for an answer and this excerpt from Field & Stream's website mentions the cycling issue:
Subsonic Rimfire Loads Offer the Best Balance for Hunting
Speaking of which, rimfires are where subsonics really shine. A standard .22 LR load delivers around 1200 fps and 140 foot-pounds of energy at the muzzle. The typical subsonic .22 LR offers around 1050 and 100, respectively, a negligible difference to any rabbit. So while the energy to cleanly kill small game remains, the downrange crack vanishes. And as smallbore competitors know, subsonic .22 LRs are more accurate than supersonic .22s because they exhibit 37 percent less wind deflection due to the disproportional increase of air resistance near the sound barrier. When subsonic .22 LR ammo and a suppressor are combined, you’ve got a whisper-quiet, deadly accurate small-game firearm.
All the major ammunition manufacturers offer subsonic .22 LR loads. CCI’s Subsonic HP, for example, reduces velocity to 1050 fps and therefore lowers the decibel level to around 68, almost half the normal high-velocity .22 LR load. (Any .22 Short cartridge will accomplish the same thing, if your gun will shoot them.) However, subsonic rounds may have trouble cycling the actions of semiautomatics reliably. If you shoot a single-shot, pump, or bolt-action rifle, you’re good to go. If you shoot a semiauto, try Winchester’s new M-22 Subsonic .22 LR, which fires a 45-grain bullet at 1090 fps and is designed specifically to work with self-loaders. In any case, with subsonic ammo, your rimfire will still be hard on game but easier on the ears.
––Jeff Johnston
source: https://www.fieldandstream.com/the-pros-and-cons-of-subsonic-cartridges/