There's an interesting philosophical question there. Can a vendor who has 15% marketshare really be called a monopoly?

Sure, they have complete control over who publishes on their own platform, but that sort of thing happens all the time without anyone batting an eyelash at it: The major console vendors do this, as did all cell phone vendors in the pre-smartphone era. My digital camera does this.

That leaves me thinking there's no real ground for invoking antitrust laws on this issue. Though there's still plenty of room to say that these policies are consumer-hostile and not in the public interest, and therefore there ought to be a law against it.