The US was asking for surrendering without conditions. That was the part that was debated by Japanese internaly, it’s all documented. Remember, their Emperor was considered to be a God. Put yourself in their shoe, what would be consequences for the population to have the US potentially trial and give death penalty to their Emperor ? Would it still be Japan? (keep in mind, people were brainwashed). In the end, even the US decided that keeping the Emperor in place was for the best, however the Japanese had no way to know beforehand, because the US wouldn’t give guarentee.