In a statement at the time, Apple explained as follows "[500px was removed] for featuring pornographic images and material, a clear violation of our guidelines. We also received customer complaints about possible child pornography."
500px, however, said that the "nude" photos on 500px weren't explicit and not necessarily the same type of nude images that Apple is famous for objecting to, meaning they're not pornographic in nature. “We don’t allow pornographic images. If something is purely pornographic, it’s against our terms and it’s deleted,” COO Evgeny Tchebotarev said.
In addition, he added that nudity on 500px instead comes from professional photographers and other photo enthusiasts, and are of an “artistic” nature. “It’s not about pornography; it’s about fine art," he said at the time.
In terms of the adjustments that 500px was required to make to return to the App Store, the company added a "report" button, which Apple requested, to make it easy for users to report offending images. In addition, 500px tweaked its search to make it more difficult for potentially offensive photos to appear based on suggestive keywords (notably, users who have opted-in to adult images on 500px's site will not be affected).
The most significant change, however, involves the app's age rating. It has been raised, likely -- though it's unclear -- at Apple's request, and is now 17+, the highest age rating in the App Store.
Since 500px was booted from the App Store, another app -- but Vine, a new app from iOS partner Twitter -- has seen porn problems. Not only have porn videos arisen, on Monday the top pick on the Editor's Choice list was a pornographic video, briefly.
Vine allows users to create six-second looping videos. For now, the app is still in the App Store, with a 12+ rating, which 500px COO Evgeny Tchebotarev noted with some degree of annoyance.