It’s not the same thing.
They aren’t asking to sell anything on the App Store, they want a free app on the App Store that allows users to connect to their service that they already pay for. They aren’t trying to sell further subscriptions, Apple are forcing them to, saying if you want a streaming App on our platform you have to make it so new customers can subscribe to it via your app, AND we have to take a 30% cut of that.
It’s Apple forcing companies to give them a share of their revenue because “App store” nothing more; nothing less. Now the 30% figure is way too high, but if MS wanted to sell subscriptions via the App Store then fair enough, Apple take their cuts but they don’t want to do that.
Your analogy with the games on Xbox isn’t the same thing. Free to play games are free to download on Xbox and there is no mandate from MS that makes sure the developers have to put money making purchases in the game.
It’s also not a digital sale, it’s a subscription service, if they lose 30% of the revenue from those subscription fees the service will become unprofitable if not loss making. Alternatively they’d have to up the subscription costs, which will put consumers off.
Just to clarify I’m really not a fan of Microsoft, Xbox, Game Pass or xCloud so I have no want to just support MS on this.