The US occupation forces and primarily General MacArthur saw Emperor Hirohito as a useful symbol of Japanese nationality. As was the trend before the war, Hirohito became something of a rubber stamp for policies enacted in his name. There was certainly some fear on the part of the Americans that trying Emperor Hirohito for warcrimes or forcing his deposition would lead to anarchy; the Japanese people were largely ambivalent on this issue as they were in the midst of economic and social upheaval caused by food shortages and a deep economic depression. It should also be noted that many in the US wanted Hirohito tried for warcrimes. It was largely due to the intervention of General MacArthur that his status within the Japanese state and society were preserved (I'm using this term loosely; the newly drafted constitution placed restrictions on the authority of the Emperor, but his function as a symbol of the state remained). During the Tokyo Warcrimes Trial, the issue of Hirohito's responsibility came up more than a few times, and witnesses, defendants, and prosecutors were pressed to carefully avoid the issue or risk tarnishing the legacy and legitimacy of the Emperor.

Source:

John Dower's Embracing Defeat.