“11.13: Apps that link to external mechanisms for purchases or subscriptions to be used in the App, such as a "buy" button that goes to a web site to purchase a digital book, will be rejected”
“11.13
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.
To be in compliance with the App Store Review Guidelines, it would be appropriate to remove the account registration links and any other fully qualified links to your site that could indirectly provide access to these mechanisms, such as links to web pages for support, FAQ, product or program details, etc.”