I think Private Slovak/Martin Sheen got a firing squad for less!
Private Slovak was subject the Law of Land Warfare (predecessor to the UCMJ). SKerry was not subject the UCMJ once he became an individual ready reservist. Reservists are only subject to the UCMJ when in a duty status.
That doesn't mean that his action in treating with the enemy were not criminal and treasonous, they were. However his reserve status doesn't enter into the question. The actions were illegal for anyone.