I had heard the condition of surrender was that the Emperor be spared.
It was supposed to be an unconditional surrender, and they had nothing left with which to bargain; our bombers incinerating their cities and vaporizing their population didn’t even need escorts by the end of the war. Many projections of the threats posed by Japan at war’s end are exaggerated; if the Allies hadn’t agreed to deal with Germany first Japan would have lost the war two years earlier (they lost the battle of Midway in June 1942). Even at the end they had a million armed troops on the Asian mainland that couldn’t lift a finger to stop the losses in the Pacific; they were an obsolete fighting force that only appeared mighty when their enemies were engaged elsewhere (or were Asian peasants).
I believe the Emperor was left in place to keep a stable bulwark against Bolshevism for the Cold War; the USSR had invaded Japanese holdings on the mainland, and we probably didn’t want them in Tokyo as well.