I think the issue is the same as Apple being anti-competitive about the AppStore. Sure, techically, both Steam and the PSN are platforms for selling games, but they’re each restricted to specific non-overlapping platforms (respictively PC and PlayStation) where there they have a (near for Steam, absolute for PSN) monopoly. Hence, anticompetitive behavior.
Wait a minute. I feel like I’ve seen that one before…
Oh well, fair enough. Humankind drew heavily on Civ in its design anyway.