Apple forces all iOS developers to use Apple's payment systems inside any iOS app and disallow the use any other method for in-app purchases. If Apple finds apps using 3rd party payment systems they will reject the app or remove it from the app store if it's already there. This has recently happened to app developer Vemedio who's Instacast app (an iOS podcasting aggregator) added Flattr support in Feb 2012 and auto-Flattr later allowing users to automatically donate to podcasts after they were playing and the update was rejected. On May 24th Apple forced Vemedio to remove Flattr support completely. The only way for apps to use 3rd party payment systems is to direct them to a website via Safari where payments can be made. Apple acknowledge that this may not provide the easiest flow for the user (as they have to leave the app, do whatever they need to do on a website, then return to the app), but it's the only way to ensure compliance. Flattr are in discussion with Apple to see if there is a solution that will satisfy Apple.