3.1.3(a) “Reader” Apps: Apps may allow a user to access previously purchased content or content subscriptions (specifically: magazines, newspapers, books, audio, music, video, access to professional databases, VoIP, cloud storage, and approved services such as classroom management apps), provided that you agree not to directly or indirectly target iOS users to use a purchasing method other than in-app purchase, and your general communications about other purchasing methods are not designed to discourage use of in-app purchase.
https://developer.apple.com/app-store/review/guidelines/#bus...
While not everyone may agree with these rules, they are clearly explained in the documentation. Releasing an app that does not follow these rules and then publicly complaining when your app is inevitably rejected is starting to seem more like a guerrilla marketing tactic than anything else here.
(Which is honestly pretty clever, I doubt yet-another-email-app would have gotten nearly the same amount of press coverage that this has gotten.)