> It would be really silly for Apple to ask the provider to censor their user's inbox and hide a particular email they received.
I agree completely. If Apple agrees, they should spell this out clearly in their terms.
As it is, Apple gets to selectively enforce this whenever they want, which is unfair to both app developers and users.
One likely danger of spelling this out from Apple's point of view is that someone might say the same thing about other messaging services besides email. Take Facebook Messenger, for example. Imagine that Facebook started offering a paid upgrade to Messenger. Why should Facebook Messenger care on which platform you're receiving an internally-generated call to action to upgrade to some hypothetical paid plan? It's not like they were intentionally targeting Apple users in this hypothetical scenario, and it should be their right to not have to censor messages they decide to send to their users just based on the platform being used. It's (hypothetically) just a regular Facebook Messenger message, after all, generated by some marketing system completely disconnected from both the app and the messaging platform!
One thing that confounds the issue for ProtonMail specifically is that the app only works with ProtonMail, and ProtonMail is operating both the frontend and the backend. It's not like the Gmail app that also lets you use other email accounts with it as a general purpose email client. This is why I chose Facebook Messenger as an example -- it is a messaging platform, but both the frontend and backend are operated by Facebook.