The post-purchase 30% cut amounts to adding or enabling content/function to the app post-purchase, meaning developers can put off charging users for some of the app until later, at which point it's fair for Apple to still get their cut - lest developers (like Epic) give away the app and effectively charge for the app later to bypass the 30% cut. E-books are, in a reasonable sense, part of Amazon's Kindle app. Movies are part of a video rental app. Power-ups and other features are part of the Fortnite app. As Apple is only charging for what is in essence part of an app, not for non-app stuff like chairs or rides or food, it's a not-unreasonable attempt to prevent developers from giving away apps and charging later to avoid the 30% App Store service charge.
Seems this is a moment that Apple, Google, and others really need to win, lest practically all apps be "free with in-app purchase to do anything meaningful".