In Short:
I think Piwik shuld report to users with the HTTP_DNT header set, that piwik respeckt their wish to don’t be tracked.
More Detailed: Piwik show two different messages to visitors on the optOut form (iframe), choosen by the existance of the optOut-cookie. So every user knows if she/he is actually tracked or not by Piwik and that it is possible to change this state right there.
But if Piwik is enabled to respeckt the DNT_Header the message on the optOut page suggest, that the user is tracked and that they have to optOut.
My suggestion on this is to implement there another new message.
We detected that you set the Do Not Track header. This Piwik-Installation is set to respeckt your Privacy so that you can’t optIn to the Web Analytics tool (and you haven’t to optOut?).
You are currently opted out by DNT header.
German Translation for the Message:
Piwik ist Konfiguriert ihre Privatsfähre zu respecktieren. Wir haben beim aufruf dieser Webseite einen Do Not Track hader erkannt, so das Piwik ihren Besuch ignoriert.
Ihr Besuch dieser Webseite wird aktuell nicht von der Pwiki Webanalyse erfasst.