The US had no way of knowing for sure what the top-level strategic decisions were in Japan. All they knew was that, throughout the war, Japanese troops virtually never surrendered, repeatedly fought to the death, and engaged in outright suicidal tactics including Kamikaze attacks. This persistence not only continued but intensified on Okinawa. There was no reason to believe that the Japanese military would ever stop short of fighting to the bloody end.

Even after Nagasaki, it took personal intervention from the Emperor and the foiling of an attempted coup for Japan to surrender.

Of course, dropping the bomb and developing the bomb are two distinct, albeit related, ethical questions.