Because unlike a lot of people, they believe this, (Thanks to Shelley Powers):
It is easy to defend freedom of speech when the message is something many people find at least reasonable. But the defense of freedom of speech is most critical when the message is one most people find repulsive. That was true when the Nazis marched in Skokie. It remains true today.
Remember this: a hundred years ago, speaking out in favor of equal rights for, oh, most everyone who wasn't white and male in this country was considered disgusting and repulsive.