He went on to say that Craigslist's government critics are trying to control the ad categories on the web site.

"Under such a radical re-envisioning, the Internet would ultimately become the province of rich and cautious media companies who would actively serve as gatekeepers to decide whether and how users could engage with the world," said Zimmerman. "The AGs are wrong to promote such a profound change ... and out of line to suggest that such a legal world already exists."

The Erotic Services category on under scrutiny since Boston University medical student and arraigned last month for allegedly murdering one woman and kidnapping and assaulting another after meeting both of them through erotic services ads on Craigslist.

On Monday, Markoff also was charged with assault and weapons charges for allegedly using a gun to threaten a woman in a Rhode Island hotel on an April 16. The woman told police that she had met her assailant through an advertisement on Craigslist.