Unpopular opinion: the 30% that Apple charges includes customer acquisition cost.
As a thought experiment, if Hey adds In-App Purchase for subscriptions, but everyone who downloads the app has first signed up for their service due to their marketing methods (an increasing likelihood considering this controversy), then nobody will use the In App Purchase, and Hey will send $0.00 to Apple.
If someone discovers the App in the App Store due to an organic search for Email clients, then they are likely to sign up via In App Purchase, and Hey pays Apple an affiliate fee of 30% of first year revenue, and 15% of lifetime revenue after that. This is a customer that Hey would not have had if not for the App Store, so getting 75-80% of lifetime value seems better than not acquiring the customer in the first place.
I realize it's not entirely as clean as that, but cast in that way, the amounts seem merely expensive, and not egregious.
(not to defend any of Apple's behavior - particularly the most recent letter sent).