Well don't forget they have SVT Rifles, which make them superior to Grenadiers.
Personally, I feel like Penal Troops are more akin to Assault Grenadiers, since both units trade their AT ability for increase base AI. Since Assault Grenadiers are 320 Manpower, 300 seems like an okay price for Penals (or for both, actually).
Only until grens get LMG or G43, then grens become vastly superior to penals.