WASHINGTON — The Justice Department used some of its most intrusive tactics against Eliot Spitzer, examining his financial records, eavesdropping on his phone calls and tailing him during its criminal investigation of the Emperor’s Club prostitution ring. The scale and intensity of the investigation of Mr. Spitzer, then the governor of New York, seemed on its face to be a departure for the Justice Department, which aggressively investigates allegations of wrongdoing by public officials, but almost never investigates people who pay prostitutes for sex. A review of recent federal cases shows that federal prosecutors go sparingly after owners and operators...