There is still a non-zero cost to implementing a checkout page, as well as associated vendor fees etc. In a fair market the fees for Apple checkout would be along the same lines as Stripe, PayPal etc., but they are using their position as the device and OS vendor to charge 10x as much and giving developers no other option.
I disagree they should be the same. Credit card fraud risk is much higher on the internet than in person for instance. Apple through iOS and the App Store I would guess, also has a significant edge in detecting and stopping fraud through name, location, etc. While 15/30% might be too high, it should be higher than a simple payment processor as the transaction is also safer for the merchant.
