Using your dominance in one market to gain advantage in another market is anti-competitive. I don't think it's cut-and-dry but I think there's a good argument that they're using their dominance in games distribution to gain an advantage in microtransaction handling.

I think Google (and Apple) using their app-delivery dominance to force app-makers to pay them a fee for in-app purchases is definitely bullshit. Consumers' options are more limited there, but that just means the market dominance is greater: the same argument applies in the case of Steam and the question is just how dominant something has to be for this to be a problem.