When New York Governor Eliot Spitzer was implicated Monday as a customer of a multistate prostitution ring, journalists rushed to brush up their knowledge of a 1910 federal law known as the Mann Act. The law, once known as the "white slavery" law, forbids the transportation of women across state lines for "immoral purposes," including prostitution. Mr. Spitzer has not been charged with violating the Mann Act or committing any other crime. It's possible that no charges will be brought at all, especially if he resigns his office. Or it's possible that if charges are brought, they will focus on...