The Apple reviewer told the company that the update couldn’t be approved because it allowed users to search for nude photos in the app. This is correct to some extent, but 500px had actually made it tough to do so, explains Tchebotarev. New users couldn’t just launch the app and locate the nude images, he says, the way you can today on other social photo-sharing services like Instagram or Tumblr, for instance. Instead, the app defaulted to a “safe search” mode where these types of photos were hidden. To shut off safe search, 500px actually required its users to visit their desktop website and make an explicit change.
500px told Apple that it would make some changes to address the issue within a day; however, Apple decided to pull the apps anyways. The app had been in the App Store for 16 months before Apple took issue with it and does not allow pornographic images only artistic nudes.
Tchebotarev says the changes promised to Apple should be done now and are being submitted immediately.