He was not charged with or convicted by court martial for the offense of AWOL. It is unfortunate, but not unusual, for officers to be allowed to resign or, if they have only a brief period of time remaining on their obligated service for it to just run out, rather than be subjected to a court martial. Reminds me a lot of the LTC in Iraq who is being allowed to resign in lieu of court martial after he held an (unloaded) pistol to the head of a prisoner and pulled the trigger-twice.