In the case of Microsoft, they denied the Xbox gaming app on the store after it had already been going through wide beta use on ios. They denied it on the grounds that they need to review every xbox game. Which is nonsense. They don't review every netflix movie, they don't review every spotify song, they don't review other content. The app that was denied doesn't actually download the games either, its a remote/online streaming player so it is exactly like "netflix for games".
Rumor is, Amazon was cut a deal and others have had a deal so the rules don't seem applied in a fair way. Amazon isn't forced to use Apple Pay for example. (which i think is the part specific with this specific case)