Is it really an "all-volunteer force" if you can't leave once you've served the term you signed-up for? WaPo:
Blocking reserve officers' resignations is one of several steps the Army has undertaken in recent years to keep soldiers beyond their original terms of service, as today's wars place unprecedented demands on the all-volunteer force. Under another practice, known as "stop-loss," thousands of active-duty Army and reserve soldiers have been temporarily prevented from leaving the military, either because their skills were needed or because their units were going overseas. As of January, more than 13,000 soldiers were being kept in the service under stop-loss, a policy criticized by some as a "backdoor draft," which the Army says it seeks to end.
If you volunteer to help a neighbor paint his fence for an hour and afterwards he forces you to keep painting for several hours more, would you still call yourself a volunteer?