Why didn't the Japanese Emperor get punished even after the two atomic bombs had been dropped ?

The Americans never promised the Japanese that they wouldn't put the emperor on trial if the Japanese surrendered. That they did not was a strategic decision in which policy would create for a smoother occupation.

The American goal was the unconditional surrender of Japan. And even though a strong case could have been made for putting the emperor on trial as a war criminal, the American viceroy, General MacArthur determined that the emperor was far more useful as a compliant tool of the Americans in aiding the occupation than as a dead war criminal. In that, he was right.

The Emperor was considered a figurehead and the Truman adminstration decided to use him as a leverage to guide them towards unconditional surrender. If he was killed, then who was going to order an unconditional surrender? No one except the fanatic Japanese officers who would assume the throne and continue on with the war as usual. The Japanese government was entirely led by the military and that was taken into consideration of how we enforce global peace. Our goal in waging World War II wasn't the punish the Emperor, the goal was for Japan to accept an unconditional surrender, that is, for Japan to surrender and disband their armed forces, for Japan to have us try their war criminals, for Japan to allow us to occupy their country to enforce the surrender terms, and for Japan to have us change their imperialistic and militarist system. The Japanese government never accepted any of these decisions and they vowed to continue fighting,

Although that can be said, the Emperor wasn't innocent, since he authorized acts that permitted Japanese personnel to commit atrocities and war crimes on a large scale, such as experimenting on human beings, excessive use of collective punishment against non-Japanese Asian civilian populations in Japanese occupied areas, and abusing or murdering American POWs.

You can read a good explanation of this in William Manchester's biography of MacArthur, "American Caesar." The US decided it would be more politically convenient to leave the slimy bastard on the throne as a figurehead while reforming the other 99.99999% of Japanese political and social culture.

No, that is not the condition laid down on Japan. The allies demanded unconditional surrender.

He was declared a figurehead. A symbol of

Japan.

The price in Japanese and American lives would have been too high.