I think this Times story illustrates the reason nicely:

That same day in Romulus, Mich., Representative John D. Dingell, a long-serving Democrat, was shouted down at a health care meeting by a rowdy crowd of foes of health care overhaul, many crying, “Shame on you!” A similar scene unfolded in Denver on Thursday when Speaker Nancy Pelosi of California visited a clinic for the homeless there.

In a statement Friday, Mr. Dingell, 83, deplored those trying to “demagogue the discussion,” but said he would not be deterred. “As long as I have a vote, I will not let shouting, intimidation or misinformation deter me from fighting for this cause,” he said.

Whenever a congressman gets roughed up at a townhall meeting full of ringers posing as concerned humble-folk, he almost always says what Dingell said: not going to deter me, I know they don't speak for my constituents, etc., etc. But, in the back of his mind, he can't dismiss the possibility that the opposition is organic. The uncertainty will weigh on him. And, spread out across a few hundred members, is likely to produce a couple dozen defectors.

But in this case it's so blindingly obvious that the mobs have been ginned up by right-wing groups and consist largely of GOP activists and insurance industry lackeys (see this footage, for example), that no congressman can honestly believe they represent majority opinion in his district. The mob-manufacturers have been poorly served by the lunatics doing their dirty work.