This makes sense to me as a general policy. Without this policy, every developer could simply bypass giving Apple a cut of their revenue by offering their app

This makes sense to me as a general policy. Without this policy, every developer could simply bypass giving Apple a cut of their revenue by offering their app through the app store for free and then charging users through an external payment system to use the application's functionality. Obviously Apple wouldn't approve such an app, meaning they should have some explicit policy against using external payment systems for any application functionality, such as the ability to read a particular book on the Kindle app. If you disagree with Apple taking a cut of app developer profits at all, that's another discussion entirely, but assuming that it's okay for Apple to take a cut of developers' profits, I think this policy is reasonable, if not necessary, new or not.