They knew of the bomb's power, but they didn't know how bad its effects of it would be in live combat and its long term effects
It was Wartime and the Atomic Bomb was a weapon, that was used in order to attain victory
Weapons are tools used to kill people, air raids or mass Calvary attacks could achieve the same results, maybe
but this ended the war instantaneously with the least loss of human life
Sure there is no justification for it, but the same could be said for War, which if it didn't break out, such a weapon would never be considered for use
Nuke's are weapons that are so dangerous that countries don't actually use them, they just threaten with them, they are so powerful they can wipe out both sides...
The U.S government warned Japan about it and they didn't understand, the nukes were simply warning shots and the casualties were unfortunately caught up in the conflict
In fact I think the old Japanese Government is also responsible for the A Bomb's use as well, if they had given up once they had been informed of the threat than those civilians wouldn't have been harmed. The A Bomb was used to force Japan's hand and give up the war
It was probably the best possible decision, not the most moral but the most efficient,
you can't equate human life but since everyone is equal you want to save as much as you can while loosing as little as you can
The Modern Japan is better for it and is an Ally of the U.S and a World Power, So I'd rather have this Japan than a Feudalistic/Japanese Empire thing go in around