Those users, however, clearly do like that 3rd party service, and would just like to get on with setting up their account there.
Any action at all by Google during that process (other than the expected one) just makes things worse for their users. The adult thing to do, then, is to simply not do anything to make matters worse.
I guess, if enough users would complain to Google or feel the need to leave their awesome free services for a more Facebook-friendly vendor, good for them. It's not like there's no choice since you can you know... export your data.