Huh? Why should we? Japan and the USA were both operating on the principles of Total War, there is no Earthly reason that we shouldn't have dropped them. The total casualties of the atomic bombings were nothing compared to the projected casualties of Operation Olympic (The amphibious invasion of Japan). I think we did Japan a sort of favor by dropping the bombs, for the cost of 500,000 people, your surrender probably saved something on the order of 10,000,000 Japanese civilians.These civilians would have died as a result of starvation, more firebombings, and suicide attacks.
Were the Atom Bombs terrible? Absolutely, and that is why we never used them after WWII. But the Alternative was near genocide, or domination by the Soviet Union. The Russians had already seized the Kurile Islands, and were pressing President Truman to give them Northern Japan.
In fact, the USA was initially planning on destroying Berlin with the A-Bomb, but the surrender of German forces caused us to focus on Japan instead.
Also, I know that the Japanese are human beings, and I know that our ancestors knew this too. The Japanese Military convinced us of your fanaticism, so we chose an alternative to a conventional invasion.