They're trying to be the last link in a chain of 90/10 (or more) splits. They repackage record labels' repackaging of artists' content. Do you think the artists would find 30% economically untenable?
The App Store is 70/30 because Apple can take things straight from content producer to customer. When the Apple takes the place of publishing, distribution, inventory, sales, payments and shipping, there's real value for that 30%.
When all someone wants out of Apple is merely to process the payment and send things down the pipe, gee, who do they think they are? But that's not what Apple is actually holding themselves out as. Apple doesn't want to be in that kind of commodity market anyway. Seems reasonable to me.