As a retailer, Apple certainly has the right to choose not to sell material that it finds objectionable. However, that's not what's happening here: In fact, Apple is denying a third-party the opportunity to sell content that Apple itself is selling.
If Apple is that concerned about adult content reaching kids, I'd argue should put the emphasis on educating parents about iOS's quite effective mechanisms for preventing kids from accessing adult material.
I'm not suggesting that Apple should open the floodgates to pornography and adult content, but it's clear that the App Store review process is badly in need of an overhaul, and that the company should align its policies for its various storefronts.
In his capacity as senior vice president of Internet software and services, Apple exec Eddy Cue oversees both the iTunes Store, App Store, and iBookstore, so the ball would seem to be in his court regarding standardizing the content allowed there. But it's not as if either the iBookstore or iTunes Store shy away from explicit content: You can find Fifty Shades of Grey in the former, and plenty of R-rated films for purchase and rent in the latter. Why should apps--especially third-party ones--be any different?
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