Maybe the act is not that different, it's the reason you give for having to do it. Japan was trying to take over and smother Pacific countries into submission and then they attacked us. I don't exactly see America as doing that. I'd be happy to let the middle east kill themselves, I don't care they're not my countrymen. The philosophy of fight them on their own land may be bull****, I'm not sure.
But at the moment the bomb was dropped the Japanese were on the verge of collapse so to say they posed a threat at the time we dropped the bomb was false.
As to the terrorist grievances, which you don't exactly see, it was the 500,000 kids killed as a result of bombing and sanctions put on Iraq. I believe they saw America as 'doing that', they probably would have much preferred we hit them with a Pearl Harbor type attack and just knocked out their military.