You cannot be terminated for being out of work due to a subpoena, but neither federal nor your state's laws require that you be paid for time spent responding to a subpoena. It is not work related in the sense that if the prosecutor subpoenaed you for a criminal  case, the state, not your employer, brought the case and is requiring you to appear--it is not at the employer's direction or instruction, and so is not considered work time for compensation purposes.