That's a licensing fee and separate to the discussion about market control. As it stands (although this is starting to change) a developer can sell the game through a variety of different avenues (online, brick and mortar, the xbox store), these avenues may have different prices associated and drive where the consumer makes their purchase.

That is the relevant comparison here, if Epic want to provide Fortnite on iOS they HAVE to use the app store and associated requirements. If they want to provide it on XBox, they pay a licensing cost for the proprietary tech they leverage from Microsoft. But they can choose to sell the game on Amazon if they want.