Because DPS is not the stat to rule them all. Performance of units is relative to faction and cost opportunity. For ex: if any faction other than USF had Paratroopers, i bet we would see them more frequently. Same with the commander/tier they are part from.
The problem i see with Penals is the fact that you ONLY get penals instead of a mix bag with conscripts. One could play with vet requirements for sure, but if more, why would i even bother going T1. And if that would be the case, why i would even play SU at that point (unless other changes are done).
I never argued that DPS is that "rules", I am simply saying that if PF vet DPS bonuses need a nerf so does Penals.
And half the issues mentioned in OP apply to Penals also.
And since you brought faction design giving Soviet's a "Riflemen" equivalent in Penals make little sense because of they completely different faction design.
Imo the fact that faction showdown Soviet played penal only and won all games is strong indication that the unit is OP. Are PF OP? imo yes simply because their performance is close to Penal's performance (who are a bigger issue atm).
If you want talk about Soviet design imo Soviet infantry should be split to defensive infantry (conscripts) and cheap offensive infantry Penals.