Google's strategy is to profit from advertising. It fundamentally benefits from knowing as much about its users as possible. Every tactic it has will always support that strategy or it will risk failure. You can tell its strategy motivates it toward evil (its term, not mine), because its slogan is "Don't be evil." It chose that slogan because it risks being evil, in its own terms. It doesn't need a slogan like "Don't…
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.