It's currently in the courts in multiple countries. The accusation is that Valve has an unwritten rule that Steam must have the best retail prices. For example, Ubisoft, who runs their own platform, could not sell Assassin's Creed on Steam for $60 and on Uplay for $50.

This would make some sense from Ubisoft's position, as they don't have to pay anyone a commission on their own store, and customers may be willing to come to them for a lower price than Steam. In this scenario, no Valve infrastructure is used for the Uplay sale.

So, again, the accusation (currently unproven, but alleged in court) is that Valve strong arms others into not being able to do this. Still it for cheaper on your own store or elsewhere that takes a smaller cut? We're gonna kick you off Steam.

This is believable, because Steam commands a dominating user share. So the threat of losing access to those users is very powerful. It's also believable because people don't become billionaires by being ethical.

However, naturally, Valve denies this unwritten policy exists. The cases are ongoing.

That is the excuse that several random valve fanboys keep making for their favorite market manipulator, but there's now like 4 separate lawsuits from different developers/producers. I don't recall any of them mentioning that.