Actually the Marine thought in good faith that he was following instructions from a competent authority about what to do with the weapon (turn it in to the armorer at Walter Reed for safekeeping) but was caught with the weapon befor he got on base. I kinda remember the original story and may be fuzzy about it, but the jury was more likely taking into account the absence of intent to violate the law and probably some sort of mistake of fact (which is a defense) as opposed to mistake of law (which is not, but given the hideous complexity these days of even the most banal laws - especially in Washington, D.C. maybe should be in some cases).
However, if this indeed was a case of a jury unanimously saying: Wha...? Why this... What the... Are you stupid? NOT GUILTY!" then GOOD.