Note the specific language being used.
"Yahoo will support the Do Not Track technology for Firefox users, meaning that it will respect users' preferences not to be tracked for advertising purposes."
The Do Not Track tag clearly specifies that the user does not want to be tracked. However, Yahoo is twisting its meaning such that the user is not tracked for advertising purposes. Two very different things. Unfortunately, despite considerable effort, there is no standardized meaning for Do Not Track. All too often, corporations invent new meanings for those simple three words in order to continue making a profit by tracking users who have explicitly indicated not wanting to be tracked. So much for notice and choice.