Or might the Soviet's decision to side with the US be because they had heard of the atomic bombs? That decision was not made in a vacuum. The Soviets' war was over, except for grabbing turf in Manchuria (which they were already doing).
When the US dropped the bomb (which the Soviets were a couple of years and some serious espionage away from) the Soviets could see the writing on the wall. By declaring war, they could lend legitimacy to the land/resources grab in progress, and let the US finish Japan. Had they declared for Japan, they would not have been our 'friends' and might have lost the opportunity to gain access to nuclear information they were stealing.
The Soviets had no other choice. Japan had no other choice, and that because of the bomb.