That's the point. You go to example.com and get the "sign in with Google" box as the only login option, but now you can't have separate uncorrelated Google accounts. Or if browsers do it automatically then every site does a background load or redirect through adtracker.nsa so you're presenting the same token on every service.

It's not the user who wants any of this to begin with. "You would never do that" except that it's now the only way to be let into the service.