>there was no military reason, so why did they do it?
Look into Operation Olympic and the expected casualties
Look at the Japanese idea to have school kids charge with bamboo spears, then see Guadalcanal and what happened to the hand-to-hand experts of the Imperial Japanese Army when the Marines showed up with Winchester Model 1897 12-guage pump actions
Check out Saipan and the civilian suicides, imagine it 50 times worse and happening on Kyushu
No idea where you get the "no military reason" from