Do-Not-Track has been designed to represent the explicit choice of the user to not be tracked. As per the DNT[1] draft :

  6.2.  User Interface RECOMMENDED
  A user agent that implements Do Not Track SHOULD provide a user interface 
  for modifying preferences.  The user interface design is left to the 
  user agent.

  6.3.  Default
  A user agent MAY adopt NO-EXPRESSED-PREFERENCE or OPT-OUT by default.
  It MUST NOT transmit OPT-IN without explicit user consent.

Another important aspect, that I don't see mentioned much, is that DNT is only supposed to prevent third-party tracking. First-party remains unaffected.

[1] http://tools.ietf.org/html/draft-mayer-do-not-track-00