It didn't. Advertisers like to think that they have a moral right to track people unless explicitly told not to stalk people. In that framework, changing the default means that a DoNotTrack header doesn't necessarily show intent on the part of the user.
Instead, the appropriate framework is that advertisers do not have a moral right to track users unless the user has consented to it. By having the DoNotTrack header be on by default, it means that a user removing it shows consent to be tracked, where previously its absence could also have indicated that the user was unaware of the header.
The default has never changed from the point of view of the user. The default is "don't steal my data". DNT was just reflecting the reality of the situation: user not making a choice indicates they don't want you to steal their data.
> DNT was just reflecting the reality of the situation: user not making a choice indicates they don't want you to steal their data.
Advertisers don't need a header telling them what they should do by default. They can get that information from elsewhere. DNT was going to be a way to opt-out, and some advertisers promised to listen to that. Setting DNT without user action removes the "opt".
Instead, the appropriate framework is that advertisers do not have a moral right to track users unless the user has consented to it. By having the DoNotTrack header be on by default, it means that a user removing it shows consent to be tracked, where previously its absence could also have indicated that the user was unaware of the header.