I think the reason is Apple are making a stand here is simply because Xbox and Google have released subscriptions games streaming services after Apple released a subscription gaming service (albeit not streaming) whereas the other exceptions were released before Apple entered its own subscription services into those respective markets.
How are these exceptions to the rule? None of those platforms are the source of the content being provided. That's the primary reason why this distinction exists.
Then what about shopping sites like eBay and Amazon? It’s not multimedia but it’s still an interactive process where you can access adult content.
There comes a point where a company’s position is so nuanced that you have to suspect it’s more to do with financial motivations than a moral stance.
And since movies are over a century old every country has a similar regulatory framework.
Most countries do not have an equivalent for apps/games and so it is ultimately left to Apple to regulate.
Also aren't movie rating systems like the MPA self-regulated? Not required by law. There's already equivalent systems for video games with PEGI and ESRB.
Still a bit flawed though: I thought that there is already a ratings board in America for software?
Every country has regulations for music/movies/TV. US and a handful of other countries have them for games and I am not aware of any for apps.
https://www.esrb.org/ratings/34178/Minecraft%3A+Story+Mode+-...
Meanwhile Netflix's own Bandersnatch is not ESRB rated, and isn't too different as a game. Yet Apple has supported both of these perfectly fine regardless of their lack of proper ratings.
We literally have ESRB that's a whole lot less arbitrary than Apple's review system.
Music/Movies are in all countries regulated by well established rules which govern what level of objectionable content is permitted i.e. you will never find porn on Netflix. Apps/Games have similar regulations in some countries but definitely not all. So Apple has opted to "fill in the blanks".