As part of its functionality it relies on the ability to uniquely identify a user across sites (to provide user authentication).
There are quite a few other services that work in a similar way.
On the other hand, if I'm not logged in, I don't want them tracking me across all the Disqus-enabled sites I visit.
As a user, I want to be stay logged in to Facebook when posting a TC comment. But I don't want Facebook to track me when I visit some random site with a like button. How do we draw the line here?
This is made easier for me by the fact that I don't use Facebook.
So Facebook & Google are fine, too then. You authenticate with them, then they add features based on that.
In the case of facebook they can show the "like" button, but they cant use the information that it has been shown for you on a particular site, on a particular date/time.
Facebook's entire purpose is to show you things you are interested in. The fact you didn't like something on a given site is nearly as useful as if you did.
Facebook show's me updates from my friends activity. It can better match those to my interests if it knows what I am interested in.
Seems that it's like P3P[0], in that it causes problems for developers but in no way keeps a company from asserting things that they don't actually follow, and there's no way to verify that they are.
Emphasis on "websites they do not visit" directly, meaning that it only applies to iframes, popups, etc. If you typed it in the address bar, or clicked on a link to get there, the site isn't limited by DNT.
This is not legal advice.