Neither app functions without a separate, non apple subscription. The main criticism of Hey, from apples perspective, is that the app doesnt just work when you download it, but the same applies to netflix.
I personally think Hey could get around it by adding some other kind of feature to the app, like a way to read their blog. When it opens on IOS you have a "sign in" or "continue with the free version" and the free version aggregates basecamp marketing copies, as long as that free version isnt just a webview, but some enhanced native experience.
Can they also advertise the cost of their service, through the applepay experience, as "x+30% apple tax"?
With how much of their app is server side (nearly all of it, minus the translation from HTML to native,) I wonder how much they can get away with, without updating the client.
The other interesting thing about this dynamic is, Apple shipped Leopard with Ruby on Rails, which Basecamp created. So in the past, Apple took Basecamp's open source code to make OSX more valuable, but now when Basecamp wants to benefits its users, at no real cost to Apple, Apple says "not without paying us our cut." Maybe not morally wrong, but at least stinks of bad karma.
Because Netflix is the dominant one in the relationship, an iPad without Netflix is a broken iPad to the sort of people who buy iPads.
and email service without an iOS app is a broken email service so Apple intends to take advantage of that.
[1] https://twitter.com/markgurman/status/1273719726196187136