I wonder if Apple's rejection of apps that track their users* will result in this API being blocked on iOS?
* http://www.tuaw.com/2013/02/26/apple-rejecting-ios-apps-for-...
* http://www.tuaw.com/2013/02/26/apple-rejecting-ios-apps-for-...
Also, what Apple was trying to combat was persistent tracking without user knowledge (done originally using iOS's exposure of device ID, which was deprecated, and now with these "cookie" tracking implementations). Allowing a user to initiate sign in to an app is a far, far cry from that (and it would be preposterous for iOS to disable the ability to log in to apps).
At least that's what apple's reviewers told me when my app was rejected.