The FBI's Pursuit of the Richard Austin Roach Thing

The Diners Club was alleging large-scale fraud and Mickey in absentia had been indicted in federal court. By the late 70s, Mickey's father was speaking regularly to the FBI. He would sometimes confront Mickey with the allegations against him. But there is something that doesn't make sense about the case against Mickey. Why did agents trace his movements on such flimsy and insignificant charges?