I agree, and the louder they yell, the deeper their predicament becomes, IMHO. They're shooting themselves in the foot, politically. Here's why:
It would be more effective to just say it's not factual (and let the viewer go in thinking it's not factual...that is if you believe BJC, but evidently lots of people do otherwise they wouldn't have elected him twice.) So the folks just think, oh, it's just a docudrama like so much on TV, and not a documentary. We saw how much impact Fahrenheit 9/11 had. (Bush is still President.)
By repeatedly yelling for censorship (and if they get it), the viewer now, even if they're prone to believe BJC, will wonder "What got cut out?" and know what they're seeing was a subject of huge controversy. By screaming for censorship, they're bringing more attention to the movie than it would have ordinarily had.