I grew up in a home where liberal politics and defense of civil liberties were thoroughly intertwined. I like to tell people that I grew up in the ACLU the way some people grow up in the Catholic church. It took some years for me to realize that there are plenty of people on the leftward end of the political spectrum who have either no great attachment to civil liberties, or only a thinly rooted attachment to same, easily dislodged. The events of the last week have provided plenty of examples of liberals and leftists whose commitment to civil liberties is fickle at most.
I'm getting tired of hearing people say that making an offensive anti-Muslim movie is "like yelling 'fire' in a crowded theater.' " Even on its own terms, that example is not exactly an unquestioned ancient legal truism. Most commentators will reluctantly accept that yes, actually using language to cause a panic with immediate physical danger to numerous people is probably something the state can prohibit and punish. Extending this example beyond literal application to the realm of metaphor is inappropriate for anyone who values freedom of speech to any significant extent.
What kind of speech needs protecting? "I love sunshine and flowers"? Only speech with potential to offend, upset, and disturb some listeners requires protection, and allowing a mob-violence version of a heckler's veto would thoroughly undermine the principle.