"it isn't bad compared to selling through conventional retail outlets"
Ignoring the fact that the internet isn't a conventional retail outlet, you'll probably notice that the price of a magazine/paper in the shop is a lot higher than if you buy straight from the source - something which Apple has specifically disallowed.
"Apple seems to be largely covering its costs"
Are you having a laugh? 30% for payment processing - and you can't really argue that you're just covering costs when you're forcing people to use your service...
The main problem is it's being forced on everyone - for those that don't need payment processing, for those whose margins are too tight to give up 30% - tough shit, use our service or no more iOS apps, and it's going to mean higher prices for us (whether we use iOS or not) and less money going to the people who actually create/provide the content.