Of course google isn't under such a legal obligation. But then they should be explicit: "We are ranking results on our own quality metric which we won't disclose, you'll just have to like them", and let the market react. This position can then be taught in schools to kids, and used whenever google search results are being evaluated by the public. Right now google brands its search results as being driven by the public.
It's like when people say google (or fb, etc) has no [first amendment] obligation to allow freedom of speech. Fine, but then don't constantly describe yourself of defenders of free speech, say from the outset you will police speech as you see fit, drop the pretense that your policing is somehow objective, and let the market digest that.