For the US Military, it is considered an honor to serve. They seek individuals who are bright, are team-players; can work well along side of others; individuals willing to take orders and follow directions; yet when needed can use their brains and initiative to accomplish a mission. Convicted prisoners by their convictions have demonstrated a lack of part or all of the above qualities that the US Military seeks. The last thing the US Military wants or needs is undisciplined misfits under arms. Please note, however, that in some other countries this is not the case. The old Soviet Union was well known for its "penal" units which were made up of convicts; many of them political prisoners. They would be given a choice; serve out a harsh sentence. Perhaps they were facing execution. Or they could "volunteer" to serve in the Army. If they, volunteered, they were almost never given any weapons but rather were used as human mine-clearers or as human shields to stop hot steel and lead that they didn't want the best troops to catch. Today the French Foreign Legion is still a military organization where a person with a criminal past can escape that past. If they honorably serve their hitch, they are allowed to become French Citizens if they wish. This is perhaps one of the main reasons why the Foreign Legion is considered to have the most brutal forms of training and discipline of all the other military organizations.