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.
Google is advertising the change extensively, and they explain it in simple English.
Google lets you opt out.
And if you were already opted out, they retain that more restrictive setting without you having to do anything.
I personally don't want to be an unpaid shill for anybody, which is why I refuse to create G+ or Facebook accounts.
But I have to recognize that Google's approach is far more open and honest than Facebook's.