Did they really? It pretty much worked as they wanted it to. Japan surrendered without a fight after that had happened.
Nixon for one wanted to seriously consider using it, and that was in 1972.
http://blog.nuclearsecrecy.com/2013/...-and-the-bomb/
Also, if the US don't want to use nukes any more, why keep so many
thermonuclear warheads stockpiled? (FYI, compared to Hiroshima / Nagasaki, thermonuclear weapons are over 100 times more powerful!)
You have 7.3 THOUSAND warheads stockpiled! Is that because they learned that they're just not worth the humanitarian costs?
They have them because they want to be able to use them if need be. And "if need be" is an ultimate last resort, one to be used if national security is that much at risk, and cannot be dealt with in any other way. The concern isn't the humanitarian losses involved with a (thermo)nuclear weapon; it's what will happen afterwards. A nuclear war can destroy large parts of your own country (expecting them to succesfully fire 'em back at ya). Game theory is what stopped the cold war from becoming world war 3, not humanitarian concerns.
I doubt it. Terrorist states would know they'd be completely decimated, but then again, they might not consider dying honorably a bad thing.