I believe his refusal to sign the form 180 has much to do with the circumstances surrounding his leaving the military.
Perhaps he was busted down in rank for his activities after he left Viet Nam and then finagled the Honorable Discharge after he became a Senator?
Wrong. Dead wrong.
Officers who leave the service received discharges with "HONORABLE" on them unless they resigned in disgrace (OTHER THAN HONORABLE) or as a result of court martial (DISHONORABLE).