Yes, that is problematic. Not by itself, but coupled with a large captive userbase it is. As an example:
Let’s say you want to start a game marketplace, which simply runs a storefront and content distribution—you specifically don’t want to run a workshop, friends network, video streaming, or peer multiplayer. Because you don’t offer these other services, you keep costs down, and can charge a 5% fee instead of a 30%.
With Steam’s policy, publishers may choose to:
List on your platform at $45, and forego the userbase of Steam
List on Steam and your platform at $60, and forego the reduced costs your platform could offer
Obviously, pricing is much more sophisticated than this. You’d have to account for change in sales volume and all. Point is, though, that publishers (and consumers!) cannot take advantage of alternative marketplaces that offer fewer services at lower cost.
The question the court has to answer is whether the userbase/market share captured by Steam causes choice (2) to be de-facto necessary for a game to succeed commercially. If so, then the policy would be the misuse of market dominance to stifle competition.
And I think Wolfire might be able to successfully argue that.