How does App Store Review Guidelines section 11.13 work?
Besides a simple "It's Apple's App Store and they'll do as they want to"...

Netflix does not provide any registration mechanism within the app. When you download the app, the first thing you get is a login screen (and a Forgot Password link). Nothing else is present.

Spotify has a registration mechanism in the app, but it also allows you to purchase subscriptions within the app. Rdio uses the same method. (As an aside, Rdio charges more for the in-app subscription than it does if you use their website to subscribe, to account for the 30% cut that Apple takes. This is allowed.)

Basically the answer is, you can provide a registration mechanism in the app, but you must also set up IAP for your paid subscription. Otherwise, you can provide a login screen for your customers who have subscribed outside of the app, but no links to your registration or subscription websites.

Look at the Review Guidelines section 11.14 (emphasis mine):


  Apps can read or play approved content (specifically magazines, newspapers, books, audio, music, video and cloud storage) that is subscribed to or purchased outside of the App, as long as there is no button or external link in the App to purchase the approved content. Apple will only receive a portion of revenues for content purchased inside the App