Advertisers make their living on loopholes and weasel words. They would have said that no matter what.
The simple fact, whether they like it or not, is that a great many people find being tagged and tracked like animals by a creepy corporate stalker to be distasteful in the extreme. Of the rest, practically none actually think being tracked is cool and even less would object to DNT being set.
The purpose of default settings is to make the vast majority happy enough. And that dictates setting DNT.
Frankly, even if you sent them a video of you signing a do not track request form literally carved in stone and have it notarized, they would claim you just didn't fully understand the issue and so your opt out wasn't genuine.