According to the article: Early this year, the attorney general of Connecticut, Richard Blumenthal, representing 40 states, sent a letter to Craigslist demandi

According to the article: Early this year, the attorney general of Connecticut, Richard Blumenthal, representing 40 states, sent a letter to Craigslist demanding that it purge the site of such material and better enforce its own rules against illegal activity, including prostitution. Craig's List voluntarily decided to curb the ads. Although they received a letter, it doesn't mean that they violated the law or were committing illegal acts themselves. US newspapers have had these types of escort ads for a long time so I'm not sure why it would be illegal to do online.