I don't know what code their sites will send my browser until they send it. But I do know that my browser tells them not to track me in its initial request, so

I don't know what code their sites will send my browser until they send it. But I do know that my browser tells them not to track me in its initial request, so they are free to send me away and not let me "in the door". But they don't, they let me in despite me being very clear that I am entering on the condition that they don't track me, so here we are. They don't get to pull the surprised-face card when I then proceed to not let them track me. Adjacent to that, I think a "I'm not going to display your ads" or bluntly "don't show me ads" header would make perfect sense and yet I'm quite sure that webmasters would treat it the exact same way that they've treated do-not-track.