I think that a lot of apparently critical history is actually authoritarian. That is, if you're going to master a population, you have to master its past. But if the population has already been educated -- or induced -- to believe that the past is nothing but a political plaything, then the question of whether the play-master is their professor or their president becomes secondary. If everyone's a critic, everyone seems free; but in fact everyone is in thrall to whoever best manipulates, with no possibility of resort to fact or truth as self-defense. If everyone's a critic, everyone's a slave.
It should be clear that Snyder isn't talking about an objective critical faculty but a sort of knee-jerk skepticism or inclination toward conspiracy theories of knowledge. Snyder seems to be saying that once we assume that history is always manipulated by historians or teachers for political purposes, we'll be tempted simply to prefer a history that suits our particular political prejudices. Hence, I suppose, both a Howard Zinn-style "People's History" and the "Patriot's History" promoted as a conservative counter-narrative. The passage arguably makes more sense if you replace "critic" with "skeptic," though Snyder's usage is probably purposefully provocative, playing on our assumption that criticism is the essential function of the free mind. But the mindset Snyder deplores is the postmodern mindset that denies objective truth in most if not all fields of knowledge, and the pseudo-intellectual paranoia that trickles down from the academy until everyone assumes "bias" in any account of events. Snyder and Judt agree that historians should be capable of teaching pure fact -- the things that actually happened. Snyder insists that objective history can convey the "underlying moral reality of [past] experiences," but a problem does arise once we concede that historians in any country have not conveyed that reality fully or accurately in the past. The practice of history in the U.S. is driven to a great extent by a felt need to compensate for past neglect of certain underlying moral realities, while a backlash has been fueled by a belief that the reformers have overcompensated to the point of neglecting more important underlying moral realities. In short, Snyder's case would be more compelling if he could demonstrate that past historians, particularly in the U.S., have practiced objective history. To prove that it's possible, you have to prove that it's been done, but many observers won't accept past proof if they perceive that neglect of certain subjects that disproves objectivity. How many facts are necessary for history to become objective? Who decides? Once you ask that question Snyder might throw up his hands, since objectivity ideally isn't for anyone to decide; it is what it is regardless of who perceives it. On the other hand, history is inevitably the telling of facts, not the facts themselves, and while democracy doesn't entitle the people's historians to lie, it does make the question of what matters subject to debate. It should be possible to debate what matters without enslaving ourselves to "whoever best manipulates," and debating what matters is criticism. I still think there's something true in what Snyder says, but I have to criticize the way he words it. At least that doesn't make me his slave.