Even if they realize there is a consensual relationship going on between newsite.com & facebook, people don't know what's going on. Does this give newsite.com the ability to see my friends? Spam them? Does this give facebook access to my newsite.com stuff?
Is 'sign in with google" like that time linkedin had access my gmail and spammed everyone?
I think that's a grayer area than phising and something more people have actually been bitten by.