That's basically what DNT was supposed to be—browsers said "do not track me" and "legitimate" trackers respected it. I agree that it's dumb. Ironically, DNT is now just one more bit that makes fingerprinting your browser easier.
DNT was supposed to be that until Microsft decided to make it on by default pretending it to be a big privacy move on their part. Because it was no longer a user choice advertisers decided it was no longer reasonable to honor it. So they just ignore it most of the time. And thus DNT became completely obsolete.
