> it's okay asking for ... a 30% cut to offer the user the convenience of paying via IAP rather than enter a credit card number
I do agree with you that it is ok (and arguably "fair") for Apple to charge a 30% fee for the convenience of IAPs. However, the real debate here is that some developers don't want IAPs, so should Apple be allowed to force them to add them?
Like the current HEY example, the developers aren't even taking money in the app, or allowing sign-ups in the app. You must be an already existing customer to use the app. But Apple is strong-arming them to add this functionality and to use these APIs that Apple gets a cut through. Should Apple be allowed to force them to add an feature to sign up and pay in the app (a function that was never planned or intended by the developers) just so that Apple can get their cut.
In OP's post he was strong-armed the same way. He had no need for any IAPs at all. He didn't even let users sign up in the app. In fact, the title of his app included the phrase "Existing users only". Yet Apple forced him to add functionality to enable IAP in the app to sign up new users, when his app is literally titled and designed around existing users and not new users. I would argue that this triggers the Anti-trust alarm bells.
Sure if you want to take advantage of the convenience that IAP provides, then let Apple charge whatever they want. If the amount is too high then developers would stop using it because it wouldn't match the value that Apple provides by their convenience. But what if I don't need in-app-purchases. Should developers be forced to add them when the app isn't even designed to use/need them? What if a developer doesn't need the convenience of IAPs and wants to take a credit card themselves? Shouldn't they be allowed to? The only argument that Apple makes in this fight is "We own the platform that ~50% of American smartphones, and if you want to participate in that market then you need to share the wealth you generate". Again, it sounds an awful lot like a monopoly.
Another example is Kindle and Audible. Apple won't let users buy Kindle or Audible books through the iOS App even though Amazon already has those credit cards associated with its' users' accounts. Kindle could enable one-click purchases (or even force you to type in a credit card every time), but Apple won't let them sell any books at all through the iOS app, citing the same rules as above.