I work on an iOS app for a company that sells educational courses, to access the app you need an account and paid subscription on the website.
Apple have recently rejected our application (out of the blue) quoting section 11.13 of the App Store Review Guidelines.
Additionally, we found that your app provides access to account registration.
As this also provides the user access to mechanisms for purchases or subscriptions to be used in the app, this is not in compliance with the App Store Review Guidelines.
While I understand the premise of this rule, I don't understand how many high profile apps with a subscription model that doesn't rely on IAP (i.e. Spotify, Rdio, Netflix) are allowed to exist in the App Store.