Key to that notion of expression is that the signal sent MUST reflect the user's preference, not the choice of some vendor, institution, site, or network-imposed mechanism outside the user's control; this applies equally to both the general preference and exceptions. The basic principle is that a tracking preference expression is only transmitted when it reflects a deliberate choice by the user. In the absence of user choice, there is no tracking preference expressed.
By automatically setting DNT when a user hits express, they have not specifically indicated they do not wish to be tracked, therefore making the setting on IE meaningless, so companies will not honor the header. IE10 does not even expicitly tell you they're turning on the setting, they just do it for you when you hit "Recommended privacy settings" which is highlighted by a large green checkmark icon.
Just look at the disparity: http://www.futureofprivacy.org/2013/12/18/tracking-do-not-tr... Compared to users on browsers that do not push DNT like IE does, only about 7% of users enable the feature, tops. Then 10x as much users have it enabled on IE10, the browser typically used by the not-so tech savvy.
Unfortunately when the browser with one of the highest market shares does this, nobody tends to take do not track very seriously, especially tracking and advertising companies who will lose their competitive edge if they choose to not track half of their userbase. It doesn't matter what Microsoft's intentions were, but the end result is they made a mockery of the standard.
EDIT: fyi, several analytical and tracking software suites such as piwik automatically disregard the DNT setting on only IE10+, while respecting the ones on other browsers. This ironically makes IE the less privacy conscious browser.
Similarly, by ignoring DNT Google and Co. just ensure that more people block their trackers and ads entirely.