This was over a year ago and I cannot find the exact text, but I found a case which looks just about the same:
Blockquote
10.6 : Apple and our customers place a high value on simple, refined, creative, well thought through interfaces. They take more work but are worth it. Apple sets a high bar. If your user interface is complex or less than very good, it may be rejected ----- 10.6 -----
Blockquote
We found the following issue with the user interface of your app:
The app opens a web page in mobile Safari for creating an account or logging in for Google+ account, then returns the user to the app. The user should be able to create an account or log in without opening Safari first.