I’d side with Apple on this if it wasn’t for the fact, as mentioned, that there are apps already in the AppStore that don’t comply with this, and Apple just doesnt care. App Store guidelines appear to only apply to small developers, if you’re a big name app (like Spotify, Netflix or Google) Apple will just bend their rules to let you in because, let’s face it, Apple needs Spotify, Netflix or Google (iOS wouldnt be that compelling for most users without those big name apps) and Spotify, Netflix and Google need Apple‘s ecosystem (and it’s installed user base). Since both parties profit from this, they’ll both just ignore the problem. The issue is that the message that it sends to other developers is that there are double standards depending on how big you are and there’s really no consistency on whether your app is allowed in or not (depends on whether Apple considers you as a big asset to the AppStore or not).
Regarding whether Apple should be entitled to a 30% cut on a service provided by a 3rd party app, using 3rd party servers and infrastructure, that’s a whole different subject (to which I’ve already expressed my view).