This makes a lot of assumptions about the business model of developers. For a BaseCamp-like product, it is mostly true, support costs notwithstanding. But a video- or music-streaming service that pays out to content providers on a per-item basis has much more in common with a brick-and-mortar retailer when it comes to profit margins.
I'm not saying it's great for every online business, just that it's also not right to see 30% and assume that's a wildly inappropriate rate.
It's also not entirely impossible that Apple realizes that the music/book/video streaming services are one of the few virtual products with material per-user costs, and adding this kind of surcharge will drive more users to their iTunes services.
This makes a lot of assumptions about the business model of developers. For a BaseCamp-like product, it is mostly true, support costs notwithstanding. But a video- or music-streaming service that pays out to content providers on a per-item basis has much more in common with a brick-and-mortar retailer when it comes to profit margins.