Because they don't know how to talk about the substantive merits when they are challenged. Having submerged themselves in disciplining each other by denouncing any heretics in their midst, they find themselves overwhelmed and outnumbered in America, where there is vibrant debate about all sorts of things they don't know how to begin to talk about. They resort to stomping their feet and shouting "shut up"... when they aren't prissily imploring everyone to be "civil."
UPDATE: Scott Lemieux, the blogger I linked to above, has written a new post in which he might think he's responding to my challenge and providing substance rather than crude denouncement. If you can slog through his laughably poor writing — "It’s not just liberals but Althouse herself who don’t believe in the standards of free speech she criticizes 'liberals' for not practicing" — you'll see he's come up with 3 links to posts of mine which supposedly show that I don't have principled free-speech values. Follow those links and see what I actually wrote.