You're actually confusing a couple of things. Advertising has very little to do with it. Say for the sake of argument they had no advertising and charged you $1

You're actually confusing a couple of things. Advertising has very little to do with it. Say for the sake of argument they had no advertising and charged you $1 per month for a subscription to use Google. They'd still have the same challenge. That is, they are privy to their users most private thoughts - more so than nearly any other business or service in life. With that knowledge comes great responsibility and risk. Employees could be drawn into schemes to provide political, social, business or criminal networks with information on what citizens are thinking. It's sensible to drive that point home on a daily basis in your culture. In a way it's reassuring that they understand that rather than be flippant about it.