erm... it was a military weaponJoke? Seriously - you posted that?
To not burn schoolchildren alive by the thousands.
To avoid bombing hospitals and killing the doctors and nurses in them.
To give the Japanese leadership time to adjust to surrender, avoid some of the risk of revolt.
To possibly - maybe - there was a chance - shorten the fucking war.
To demonstrate moral courage on the eve of ascending to the leadership of the free world.
you're applying your ideology to the situation in hindsight
the whole "military warning the other side of impending attacks" is nonsensical in light of the declared war and the concerted effort to fight - the Japanese didn't warn us of Pearl Harbor or of their Kamikaze attacks. it was war. sh*t got dead using military action.
it was already planned for huge losses -ours and theirs- should we have to invade the mainland. The Japanese were willing to defend it and predicted how we would attack.
There is no concievable way the Japanese would be willing to accept a single bomb that destroyes cities at a single detonation as being a viable working weapon (in that day especially)
it would be like Iraq telling us they built a deathstar that is cloaked in orbit