Before, the App Store guidelines said the following:
Apps can read or play approved content (magazines, newspapers, books, audio, music, video) that is sold outside of the app, for which Apple will not receive any portion of the revenues, provided that the same content is also offered in the app using IAP at the same price or less than it is offered outside the app. This applies to both purchased content and subscriptions.The new guidelines change this completely, eliminating any requirement for offering the same price and the requirement to offer external subscriptions as in-app purchases:
Apps can read or play approved content (specifically magazines, newspapers, books, audio, music, and video) 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 not receive any portion of the revenues for approved content that is subscribed to or purchased outside of the app. Read More