I think Pierre Vidal-Naquet got it right when he wrote:
"The simple truth, Noam Chomsky, is that you were unable to abide by the ethical maxim you had imposed. You had the right to say: my worst enemy has the right to be free, on condition that he not ask for my death or that of my brothers. You did not have the right to say: my worst enemy is a comrade, or a 'relatively apolitical sort of liberal'. You did not have the right to take a falsifier of history and recast him in the colors of truth.
"There was once, not so long ago, a man who uttered theis simple and powerful principle: 'It is the responsibility of intellectuals to speak the truth and to expose lies.' But perhaps you know him?"
Brad DeLong