True, there is nothing stopping them from running this in the background to make additional linkages between people and places to sell to third parties without having to share where those linkages come from.
Plus if you opt out you're only opting out of the social aspects of this recognition, not the revenue models behind it.
Thank you for pointing this out. They don't claim that they won't recognize your face if you opt out... they just won't suggest it to your friends.
Yes, here's an article describing the feature from 2010.