I see it partly as a response to the fact that the industry is moving from one-time purchases to subscription-billing. Apple were always able to make their cut from the one-off sale of a $34.99 premium application. Now that the app is available for a $2.99/month subscription from the developer, Apple have to recover the lost caused by the shift to subscription models somehow.
And they certainly do have costs associated with content delivery. There's the trivial costs (bandwidth, hosting, etc.), and the larger overheads associated with R&D, developer review, continually fighting security risks, and so on.