it’s x percent but you’re using our product and infrastructure and we have to invest people and resources to verifying apps getting published.
If Apple wanted, they could allow developers to supply their own infrastructure. It would cost a lot less than 30% of the developer’s gross revenue. Apple could also charge, say, ten thousand dollars per hour for the time the review team spends checking the app. That would also work out to astronomically less than 30% of the developer’s gross revenue.
The App Store is a great service and Apple is entitled to collect a fee. But the amount they’re charging is excessive. In a fair market with proper competition they could never get away with charging that much.