It's time to stop asking "Why does Apple think they deserve 30% of App Store revenue" and start asking "Why has Apple not demanded 30% of web app revenue yet?". The justification of "We put all the hard work into building the hardware, the OS and the stack so deserve a cut" still stands if a website is opened using the Safari engine. If it doesn't stand then neither does the App Store cut.
A buyer gives $x because they are betting they cannot buy at $x-1 at whatever scale they want to buy at.