30% makes sense for an app. At least 30% of what's running when you run an app is library code, provided by the OS vendor. Apple write the code to recognise gestures, draw on the screen, play the sounds, etc. And since OS upgrades are free, something has to cover the cost of that. But when people are just downloading additional media content, and Apple aren't even hosting it, the 30% charge doesn't make as much sense.