Not suggesting that this warrants a 30% cut, but unlike payment gateways Apple also provides discoverability and distribution.
Apple seems to be incredibly poor at discovery, so that's not a point in their favour.
Distribution could be done by any number of other CDNs, as will likely be done by the new EU based App stores.
What's the bet those won't be charging ~30% transaction fees to cover things? And they won't even have the same scale as Apple. ;)
Not suggesting that this warrants a 30% cut, but unlike payment gateways Apple also provides discoverability and distribution.