Classified advertising website Backpage filed a lawsuit against an Illinois sheriff's office on Tuesday, alleging the sheriff had convinced major credit card companies to stop processing payments for listings on the site, therefore violating the free speech rights of its users. The suit comes less than a month after Cook County Sheriff Thomas Dart sent a letter to the CEOs of MasterCard and Visa, in which he argued Backpage plays a role in furthering sex trafficking in the U.S. by hosting thousands of ads for prostitutes…The company is seeking damages to make up for the lost revenue from ad postings, as well as compensation for the value of lost goodwill and punitive damages…Liz McDougall, a lawyer for Backpage.com LLC, told The Wall Street Journal the goal of the lawsuit "is to ensure that one elected official, particularly a county sheriff, cannot dictate what speech is or is not appropriate."...