(2) Each Government pledges itself to cooperate with the Governments signatory hereto and not to make a separate armistice or peace with the enemies. --From Declaration by United Nations (1942)
That might explain it even if the US was military for pursuing armistice/peace without using the bomb the treaty clearly spells out that the UN joint operational policy would over-ride US law or policy with respect to prosecution of war with Japan--the US was from 1942 onward obligated to prosecute war the UN-Joint way.