As far as I know, DNT was designed to be a tri-state with { NoPreference, On, Off. NoPreference is the default. If it is turned on by default, what would NoPreference mean?
One could argue that DNT preference where chosen when the users opted to use IE with DNT as default. As such, NoPreference has no meaning when the user chose is always made one way or the other.
In the end, Microsoft made the decision to force it into a yes/no, rather than leaving it at "NoPreference". I can fully see the argument that Microsoft is not following the spirit of the standard in doing so.
