from www.nytimes.com – The sheriff of Cook County, Ill., grabbed headlines earlier this year when he sued Craigslist, the online classified advertising forum, for allowing posts that he said promoted prostitution.

A federal judge in Chicago wisely threw out the suit last week. As Congress has recognized, if an Internet proprietor had to police every posting that a third party put up, the cost would be enormous — and it would likely stifle communications.

Craigslist warns users that offers or solicitations of prostitution are prohibited. Sheriff Thomas Dart [pictured] argued that its “erotic services” section still included numerous listings for paid sexual services, including some using code words. The company made voluntary changes after the suit was filed, including conducting a manual review of the listings. Late last year, before the suit was filed, it started charging for those ads in an effort to appease critics.

Even without these changes, Craigslist was operating entirely within the law. The Communications Decency Act of 1996 protects “interactive computer services” — ranging from small bloggers to giant Internet service providers — from liability, in most cases, for speech they did not help create.

The legal question before Judge John F. Grady was not a difficult one. Last year, the United States Court of Appeals for the Seventh Circuit, whose decisions are binding in Illinois, ruled in a fair-housing case that Craigslist cannot be held liable for its users’ illegal real estate listings. As Judge Grady rightly concluded, the same logic applies to adult listings.

Other law enforcement officials, including several state attorneys general, have attacked Craigslist recently for its adult listings, despite its immunity under the Communications Decency Act.

This is the wrong approach. Sheriff Dart told the court that his office had conducted sting operations using Craigslist that led to numerous arrests on prostitution and related charges. He seemed to think it was an argument against Craigslist, but it actually shows why suits like his are unnecessary.