Sorry... wrong. They are WAY more than payment processing. iTunes and the new Newstand is a channel. It's easy access to millions of potential customers and that is worth the 30%.
No, it's not worth it. Especially for digital retailers. COmpanies like Netflix or Amazon make less than 30% out of each purchase. So you're bassicaly saying it that just giving access to customers is worth more than everything those companies are doing? And that they should sell for loss just because of how great of a platform Apple has created? Suuure..
Of course, this won't be a problem now that there's no rule to have the lowest price avaible on iOS. THis way even with in-app purchases those content providers can just rise the price up for iOS version enough to make a profit.