Imagine if Microsoft did this on PCs. a) prohibiting the installation of non-windows store software (sideloading) and b) insisting that all purchases done via apps give them a 30% cut. I think this is a ridiculous practice on the behalf of Apple.
The most egregious part of Apple’s rules, and the reason that online service providers have a special loathing, is that apps are disallowed from linking to, advertising, or even mentioning that it is possible to sign up/subscribe/buy/rent outside of the app.
This is why you won’t see MMOs like FFXIV through the App Store, and is why you can’t sign up for Netflix, or even follow a link to their sign-up, from within the app.