I want to note that the persistence of quasi debtors' prisons afflicts the military legal system as well as civilian ones. I have held civilian positions with both the U.S. Navy and Air Force JAG Corps and will be beginning a two-year clerkship with the Court of Appeals for the Armed Forces this coming Fall.
Under the Rules for Courts Martial, fines may be imposed on those convicted of criminal conduct under Rule 1003. In turn, under Rule 1113, it is outlined how to proceed with "Confinement in Lieu of Fine" provisions in sentencing. Military judges may include language in sentences stating that any criminal fine will be enforced through confinement should the convicted fail to pay. The confinement is to be of a duration equal in severity to the fine (although how to accurately translate a fine into confinement is not clarified). The language of the rule appears to allude to any criminal fine of this nature replacing confinement, but high military courts (particularly the Navy-Marine Corps Court of Appeals) have interpreted this to mean that the confinement is simply a repercussion for not paying the fine and does not replace the fine as punishment for the criminal offense. This leads to the incredible situation where someone can be confined for not paying a criminal fine for a duration considered equal to the fine and then still owe the fine despite serving time behind bars. Since the confinement is equal to the fine, and the fine was deemed an appropriate punishment for the criminal offense on its own, the convicted individual is essentially being slapped with multiple sentences that each on their own are considered punishment enough for the crime.
The above situation appears to be patently unjust. I think it is important to pay attention to the military legal system as well as the civilian ones since the former actually has jurisdiction over more people than some state criminal codes given the population of the armed forces.