That's the argument I've heard from the ad industry, but I don't really buy it. Previously, the DNT header was missing by default, so it's presence could be seen as user intent to forbid tracked, while its absence is ambiguous. Afterwards, the DNT header was present by default, so its absence could be seen as user intent to allow tracking, while its presence is ambiguous. That's exactly what should be the case should be, where only explicit consent to be tracked counts.