Hirohito was presented as a god to the people of Japan and they believed so too. He was the one and only leader for them. Since Tojo Hideki was mostly in charge for the military and its doings and Hirohito was most likely a puppet of him and the military, Hirohito was spared by the US and not accused of anything since there was no evidence that he gave any kind of major order to the military and its operations. Tojo was brought to justice and took everything the japanese military did on him, with the over 4 million casualties the Japanese have caused. There is proof though that politicians and such destroyed documents that could endanger the Tenno in any way and they said that they have given the commands and not him just to spare his life. The US military spared Hirohitos life because of that and beceause he was the one that pulled the country together and was the only one that could lead Japan into a new age out of imperialism since the people worshipped him like a god and did what he said.