> You keep saying they are enforcing 'the law,' when in fact they are just enforcing their own TOS, which is perfectly within their rights.

Relevant links:

http://en.wikipedia.org/wiki/Defamation

http://en.wikipedia.org/wiki/Vigilantism

There are laws regarding defamation, I don't think DO can just opt in to enforce them. They can write anything in their TOS, wether or not it's legally enforceable is another question.