If Americans killed the Emperor, and then the vengeance upon American servicemen caused a shift of the US war effort to the Pacific Theater, what would the Soviets do? Might they want to start improving relations with Japan? The main Western "ally" just left them out at the mercy of a still powerful Reich.
What’s the benefit to allying with a country in serious danger of getting bombed into the Stone Age? They won’t exactly have much to offer.