This is a privacy concern and a step backwards for both Piwik and DoNotTrack in general.

Instead, allow the user to choose at install or configuration time whether or not to honor the setting, with a clear statement of why each way, such as the following example text:
Respect DoNotTrack Preference?
It is encouraged to respect user privacy choices by respecting the DoNotTrack browser privacy setting. Please be aware, though, that IE10 has it enabled by default and some traffic may not be recorded.
How do you want to handle this?

[X] Respect DoNotTrack in all situations (recommended)
[ ] Respect DoNotTrack for all browsers except IE10 (for more accurate stats)
[ ] Do not respect DoNotTrack at all (not recommended)

Also, DoNotTrack should be able to be set per-site, rather than globally.
Keywords: DoNotTrack