Its perhaps worth noting something of what George MacDonald Fraser said in his autobiography (author of Flashma books, he fought as a private, then a lance-corporal in burma during wwii. The book, which is entirely account of his time in Burma, is called "Quartered Safe Out Here" and I recommend it to anyone):
-That the Japs were absolutely determined not to surrender, despite the hoeplessness of the situation (I've read several places that Japanese forces on the continent refused to believe that Japan had fallen, and fought on for TWENTY YEARS).
-That at the very least, aside from the huge numbers of troop casualties that an invasion of Japan would have cost, any invasion of the mainland would have seen those Japanese still fighting on continent would have gone crazy, and executed the tens (hundreds?) of thousands of prisoners under their control (Supported by King Rat - ficton, but loosely based on James Clavell (the author) and his experiences in a Japanese conc. camp at the end of the 2nd world war).
-THAT ANY DISTINCTION BETWEEN CIVILIANS AND SOLDIERS IS MEANINGLESS, when conscription ensured that many "civilians" were fighting in the army. Suggesting otherwise is also a view which inherently weighs numbers of lives of soldiers vs. non-combatants, and somehow finds the life of a soldier less worthwhile than that of a non-combatant, and for the life of me, I can't see why.
-That in a war of aggression, that the aggressive (and then defeated) country suffers the brunt of the casualties is perhaps justified.
-That US money for munitions was low, and increasingly difficult to acquire, to the extent that the war in Japan may have become deadlocked.
-That the feeling at the time was effectively "Jolly good, we've finally won, and we can stp being scared of booby-traps and suicide attacks, and go the heck home." There was, perhaps understandably, little sympathy for the Japanese.
It's very easy to judge what was certainly a horrific act, with an immediate, identifiable effect, whilst at the same time its difficult for modern sensibilities to conceive of the viciousness, determination and stubborness of Japanese troops, and the possible casualty numbers that would have resulted from a lack of bomb dropping. It's also worth noting that Fraser, with all his prejudices, was a normal, humane, and emminently human man, fighting in a war not his own, and that his feelings were those of many of his contemporaries.
What it isn't, is fair. You can go on and on about humanitarian costs NOW, but that doesn't change the fact that in a war, the aim of each side is to make sure as few of your men die as possible, whilst achieving victory. The atomic bombs did this, in a way that would not have been achievable without further high costs to British and US forces.