The problem is the pressure on the recruitment offices. They have to meet quotas. Branches like the Army or the Marine Corps would take any able bodied young person, even if they have a criminal record(repeat offenders) back in the days. The Marines , whom are always looking for a few good men, took anyone in. I am sure that the rigors of boot camp can change people(when I was in the NJROTC, I experienced one week of boot camp in the Orlando Naval Training Base). And I am sure that any of the branches would take anyone who has honestly admitted to the recruiter that he or she has a record, military service would be preferable to prison or the grave.
The Department of Defense had been investigating this problem for a while but have found no real solutions.