Actually they don't.
Military or Overseas Voters
The Uniformed and Overseas Citizens Absentee Voting Act (UOCAVA) created special provisions for absent uniformed services and overseas citizens to vote by mail in primary, general, and special elections for federal offices; and to allow these individuals to use a federal write-in absentee ballot (FWAB) in the general election for federal offices.
Educate your self.
It is just another way to rig the voting process.