First, everybody cares about privacy. Just because businesses have taken advantage of users being clueless, doesn't mean those clueless people don't care about privacy.
Second, your "nobody knows about it" argument can cut the other way. How can a business track people when most people don't know/care that they can opt out of being tracked?
Nobody wants to mention it, but the truth is that if it were on by default, few people would turn off DNT once they knew what it was for.
I don't like MS, and I don't even own a computer that can run IE10, but I think this is a great idea.