It doesn't allow them to track, but it does allow them to more convincingly argue that they can nag them about it (I think some regulators in some EU countries have rejected this, but I don't think this is universal). i.e. it makes it ineffective as a means of stopping the annoying pop-ups. Because the companies are basically belligerent about it there needs to be a clear declaration of 'if this header is set you may not track _and_ you may not bug the user about it'