Let's assume Aunt Agatha reads about the DNT setting in one of her magazines, goes to the browser settings, sees that it's enabled, takes no further action. That is no different from Uncle Ulysses who has a different browser with a DNT-disabled default who goes to the settings and enables its.
My letterbox has a sticker on it that I don't want to receive unsolicited ads. I don't need anyone to try to presume that it wasn't me who put that sticker onto my letterbox.