First, for all those wanting life imprisonment or calling this a miscarriage of justice need to realize that the military and the US Army in particular were holding all of the cards.  This was a military court, not a civilian one so it was their decision.  It would be a pretty safe bet that the Colonel who was the judge was most probably made aware of the "Army's" preferred outcome.  It's obvious to me that the Army didn't want to pay for him any more (particularly with his health problems) or deal with any further press on the subject.  They just wanted him to become 'not their problem'.
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Second, the UCMJ doesn't have an article for treason.  The closest is probably Article 104 "aids, or attempts to aid, the enemy with arms, ammunition, supplies, money, or other things;" which doesn't apply so that was never going to be charged.