Why isn't Do Not Track enabled in Tor Browser Bundle by default? I'm aware that it's ultimately up to each site to decide if they'll honour the request, but wouldn't it be sensible to go ahead and enable it for the sites that do?
This should not be considered an authoritative answer, but my best guess is:
- Most users do not have Do Not Track enabled. So enabling it makes your traffic stand out and makes you more uniquely fingerprintable. As bastik points out in another answer, the Tor Browser maintainer seems to agree.
- Do Not Track is technically advisory only; web sites are under no technical obligation to honor it. And even though there may someday be legal obligation, that can be set aside whenever it's convenient for those who make the laws.
So, in short, it's unnecessary to enable this, and doing so may actually reduce your privacy.
Additionally to what Michael Hampton said the maintainer of the TorBrowser is not in favor of DoNotTrack (DNT), because it is just another bit to fingerprint on and might not be honored at all as it is privacy by policy, where privacy by design approaches really work.
For me personally it is also a short-coming of DNT, that it is implemented as black, white or gray. I couldn't allow a website to track me if I want to block some others, or the other way around.