> Given those goals that they had, why would anyone expect them to behave 'ethically' or care about the side effects of their activity ?

Because the bomb was meant to be used (in "production") on foreigner civilians. And that was (and still largely is [0]) acceptable to Americans for a variety of reasons: "the Japanese were barbaric war criminals", "they did Pearl Harbor", "mass murder by atom bomb was compassionate given that the only other alternative was mass murder by conventional bombing/invasion", etc.

In contrast, the testing affected the health of law-abiding Americans. That's very un-American.

[0] http://www.gallup.com/poll/17677/majority-supports-use-atomi...