As industry standards evolve, how we implement those standards evolve as well. So to reflect the current requirements of the privacy standard for tracking preferences, Microsoft is changing how Do Not Track (DNT) is implemented in future versions of our browsers: We will no longer enable it as the default state in Windows Express Settings.

While our implementation of DNT two years ago in Internet Explorer 10 (IE 10) was welcomed by many, others voiced concerns, especially given that discussions were underway at the time to establish an industrywide standard for user tracking preferences.

Since then, the World Wide Web Consortium (W3C) has continued to refine language to address how users express a preference regarding tracking. The latest draft of the standard reads:

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. (Emphasis added.)
Put simply, we are updating our approach to DNT to eliminate any misunderstanding about whether our chosen implementation will comply with the W3C standard. Without this change, websites that receive a DNT signal from the new browsers could argue that it doesn’t reflect the users’ preference, and therefore, choose not to honor it.

As a result, DNT will not be the default state in Windows Express Settings moving forward, but we will provide customers with clear information on how to turn this feature on in the browser settings should they wish to do so. This change will apply when customers set up a new PC for the first time, as well as when they upgrade from a previous version of Windows or Internet Explorer.

We said in 2012 that browser vendors should clearly communicate to consumers whether the DNT signal is turned off or on, and make it easy for them to change the setting. We did that for IE 10 and IE 11. And we’re continuing to do so with future versions of our browsers.
Source: An update on Microsoft's approach to Do Not Track - Microsoft on the Issues