Couldn't they offer an app with no in-app "buy a subscription" option and avoid paying that Apple tax? I guess added value for existing subscribers isn't very appealing on its own.
There is an existence proof that that isn’t the case. Spotify, Netflix, Sling, and DirecTVNow are all in the App Store and none of them allow subscriptions from within the App Store.
That is simply proof of selective enforcement. your examples have HUGE preexisting User Base, should apple ban them it would have a real effect on Apple Adoption rates
They are big enough to make apple bow, other apps are not like that