One good reason why it "shouldn't" do this? It's doing it and giving you a choice to either be a part of it or not. That is more than you can ask for from a free service. If you were able to be a part of a service, not pay a thing, feeding data to it and expecting it not to use it in anyway you wouldn't like- then you would also not see the constant innovation brought about by such companies; and this is justified as long as the user is not forced into it; the golden rule of software.