Does it set a precedent that I can send a header "do-not-show-ads" and they can't send me ads?
It would be really fun* if the headers of an HTTP request were to be considered part of a legally binding agreement between client and server.
*In a "mentally deranged fun" sense, of course.
It's not a crime to put an advertising truck outside a house, but it can be to put a (private company) surveillance truck.