A New Jersey federal judge is scheduled to consider today whether federal law preempts a state statute designed to crack down on online classified ads for underage prostitution. (Update: the judge has enjoined the law, holding that it likely conflicts with federal law and the First Amendment.) Some anti-prostitution activists charge that sites such as backpage.com are knowingly profiting from the practice. But free speech advocates argue that efforts to censor the sites will undermine online freedom of speech more broadly.

At the heart of the controversy is a provision of the 1996 Communications Decency Act, which made it a crime to send "indecent" communications to minors. Ironically, while the Supreme Court in 1997 struck down the provisions of the CDA that restricted online speech, it left standing Section 230, which immunizes Internet service providers from liability for content posted by their users. Section 230 is beloved by free-speech advocates because it lets people operate uncensored online forums without worrying about being sued for their users' posts.

But the attorneys general of the 50 states are frustrated by Section 230. State officials have been pressuring the classified service backpage.com to shut down its "adult personals" section, which is sometimes used to advertise underage prostitutes.

Backpage has refused, and the courts have supported the site. In 2011, a judge cited Section 230 to dismiss a lawsuit against Backpage by a woman who had been forced into child prostitution. Last year, a court blocked a Washington state law requiring companies to verify the ages of the subjects of online prostitution ads, holding that the law was preempted by Section 230.

Two weeks ago, the National Association of Attorneys General sent a letter to key members of Congress asking for Section 230 to be revised. Right now, Section 230 immunizes Web site operators against civil and criminal liability for their users' speech. The NAAG wants to narrow that so Web site owners are protected only against civil lawsuits, not criminal prosecution.

The NAAG letter provoked a response from a broad coalition of civil liberties groups, high-tech investors and legal scholars. They worry that narrowing Section 230 will undermine freedom of speech online. "Section 230 is the legal cornerstone of the Internet economy, enabling the unprecedented scope of lawful commerce and free expression that the Internet supports today," they wrote in a letter to congressional leaders. The coalition warned that modifying Section 230 would "jeopardize the continued growth of the entire Internet industry and the free expression rights of Internet users everywhere."

Without Section 230, they warn, "online service providers would expose
themselves to potential prosecution under literally thousands of criminal statutes on a state-by-state basis. Keeping up with the thicket of state criminal laws would be a significant burden, especially for start-ups and smaller companies."

Kevin Bankston, an attorney at the Center for Democracy & Technology, argues that the states' focus on Backpage is misplaced. "Backpage cooperates with law enforcement in helping them find these people," he says. He points to a recent 70-city raid that "resulted in the rescue of 105 teenagers and the arrest of 159 pimps." That operation was made possible because Backpage supplied law enforcement with information about who had posted ads.

But Jill Habig, a legal adviser to California attorney general Kamala Harris, argues that merely cooperating with law enforcement isn't enough to exonerate Backpage for its role in child prostitution. "In criminal law, you have liability for someone who is an accomplice to a crime," she says. "Backpage is used to profit from the sale of human beings."

Bankston counters that there's little reason to think Backpage would be found liable as an accomplice to prostitution even without Section 230 on the books. In his 2011 ruling holding that Backpage was protected by Section 230, Judge Thomas C. Mummert also found that the plaintiff did not "describe the specific intent required for aiding and abetting." Bankston says that "even if [CDA Section 230] hadn't existed, that case would have been dismissed."

Similarly, Bankston said, the Washington age-verification statute probably would have been struck down on First Amendment grounds if Section 230 wasn't on the books.

"Although they characterize this as a narrow fix, this is an incredibly broad proposal for a very narrow, specific problem," Bankston says. "We're talking about opening [online service providers] up to liability for a wide range of criminal defamation statutes, state privacy laws, a practically unlimited array of potential liability that would send any sensible investor running away from any kind of user-generated content play," Bankston warns.

Most important, Banston argues, exposing Web sites to liability for their users' speech "would be devastating to the free expression environment online." Section 230, he says, "is as important to free speech online as the First Amendment."