Although Craigslist is privately owned, it is very ubiquitous.  In order to keep its main properties "respectable," it will respond to governmental pressure - think BP and Gulf of Mexico.
otoh, read closely this NY Times article, and note that CL was not bowing to any LAWful argument.
http://www.nytimes.com/2010/09/05/te...ml?_r=1&ref=us
if the above link doesn't operate, here's the entire article:
September 4, 2010
  
Craigslist Blocks Access to ‘Adult Services’ Pages
     By CLAIRE CAIN MILLER
              Craigslist, the popular Web site for classified ads, has blocked access  to its “adult services” section and replaced the link with a black label  showing the word “censored.”   
     
  Law-enforcement officials and groups that oppose human trafficking have  been highly critical of Craigslist, saying that the adult ads helped  facilitate  prostitution and the selling  of women against their will. 
  Craigslist, which is based in San Francisco, did not respond to requests  for comment, and it was unclear whether the block represented a  permanent shift in policy or a temporary protest against the outside  pressure on the company, which has lasted several years. 
  Last month the attorneys general from 17 states sent a letter to Craigslist’s chief executive, Jim Buckmaster, and its founder, 
Craig Newmark, asking the company to immediately remove the adult services section.  
      
  The controversy is the one of the most prominent in the debate over free  speech on the Web, where anyone can easily and anonymously post  anything: just how much responsibility does a Web site have for what is  posted by its users, or for potential criminal activity that results  from the posts? 
    
  The company, while promising to provide more rigorous oversight of the  ads, has defended its right to run them and says it is protected under  federal law — the Communications Decency Act — a position that judges  and legal experts have generally backed. 
“They can absolutely keep it up. The law is pretty crystal clear on  this,” said M. Ryan Calo, a senior research fellow at the Stanford Law  School’s Center for Internet & Society. “What’s happened here is the  states’ attorneys general, having failed to win in court and in  litigation, have decided to revisit this in the court of public opinion,  and in the court of public opinion, they have been much more  successful.” 
  
Richard Blumenthal,  the Connecticut attorney general who helped lead the effort against  Craigslist, said by phone on Saturday that “these prostitution ads did  not promote a victimless crime. There is human trafficking in children,  assaults on women.” 
  He said he was pleased that Craigslist appeared to be “doing the right  thing voluntarily” but added that his office would continue to monitor  the site and was trying to determine if Craigslist was closing the  section permanently. Craigslist continued to block access to the section  on Sunday. 
  The ads in the adult section, which cost $10 to post and $5 to repost,  are a big revenue source, analysts say. Craigslist  is private and does  not report financial figures. But adult ads are expected to bring the  company $45 million in revenue this year, according to the Advanced  Interactive Media Group, an organization that analyzes Craigslist. 
  Some Internet law analysts said on Saturday that Craigslist could be  sending more than one signal  — that it was both capitulating to law  enforcement and thumbing its nose at it. 
“There are multiple ways in which to censor speech — one is directly  through the courts, and the other is through a form of protest that  says, even if you can do this, stop doing it,” said Thomas R. Burke, a  lawyer at Davis Wright Tremaine who specializes in Internet law and is  not involved with Craigslist. “Maybe their point in saying they were  censored is that people need to understand the law better.” 
  But Malika Saada Saar, executive director of the Rebecca Project for  Human Rights, a nonprofit group that has urged Craigslist to shut the  adult services section and screen the entire site for such ads, said the  company should be held responsible for what appears on its site. She  said Craigslist “has the legal responsibility as well as the moral  responsibility” to close the section. 
Craigslist has taken steps to  appease critics before. In May 2009, it removed its “erotic services”  category and replaced it with “adult services,” for “postings by legal  adult service providers,” and had all adult services ads manually  screened by a lawyer before posting. 
But criticism has continued, fueled by prominent cases like that of 
Philip Markoff,  a Boston medical student who was charged with murdering a woman he had  met on Craigslist. He pleaded not guilty, and he died in jail last month  in an apparent suicide. 
  The section in question appears not to have been blocked abroad. In  France visitors to the site have access to the Érotique link and can see  material intended for adults. 
= = = = 
Case law does exist now in re the Communications Decency Act (of all things, very rightly named in this instance) and most recently the suit by the Sheriff of Chicago against CL which was tossed out by a federal judge in Chicago last year who cited the Act in doing so.
Attorneys General are politicians and will always respond to the squeaky wheels (citizen complaints) and try to make hay with publicity.  Several AGs, (think Spitzer) played that game very well until, well, until the cookie jar turned out to be a publicity trap, too.