Japan was already in the process of trying to surrender as the Russians had dropped the neutrality agreement. Their only requirement was that the Emperor be allowed to remain in his ceremonial position.
That’s not how surrender works. The victor is the one that sets the requirements, and the loser meets them.
You’ve nailed how unconditional surrender works.
Sometimes you can get an enemy to surrender sooner, at lower cost to everybody, by agreeing to some terms they set. While still, of course, keeping the large majority of terms in your own favor.