But it doesn't necessarily have to be considered 'public data' for Google to be able to exploit it. Google uses opt-in by default for privacy settings and has a

But it doesn't necessarily have to be considered 'public data' for Google to be able to exploit it. Google uses opt-in by default for privacy settings and has a long track record of being misleading when it comes to privacy. I'm sure there's plenty of people out there who have unknowingly opted in to 'volunteering' their Gmail data or will soon be.