They should add the National Association of Realtors to that list. They may not have signed the letter in fact, but they apparently support censorship in principle and action.
The 800-lb legal gorilla of the NAR jumped on my own back, only yesterday. I set up a site for an audio drama I recently produced -- a fun little ghost-story for geeks, which happens to lampoon the Realtors and high-tech CEOs of Silicon Valley, whom we all love to hate. My URL corresponds to the Title of that fictional story, "The Realtor and the CEO" (http://www.realtorandceo.com). They decided that they did not like my using the word realtor as part of a literary title, and are now trying to coerce me into giving up the URL, the Title of the audio drama, and any reference to realtors in the story -- which happens to require eliminating or completely rewriting a main character.
Seems First Amendment rights mean nothing, if you do not have a $100,000 war chest.