One could argue that since Apple must provide storage, infrastructure, and man-hours for all the apps that are purchased for iOS devices, that they are well within their rights to take a cut of App purchases. But what justification do they have for demanding the same regarding content consumed by said apps? They do not store the content. They do not filter the content. They do not provide the bandwidth that delivers the content. They are not liable for what the content itself is. What possible justification could they have for doing this?
This is very nearly the same as saying that Sony should get a cut of all ad revenue for anything viewed on their TVs.