More of a history question than a science question.

I mean, Western nuclear powers tested pretty close to/deliberately on their own troops and civilians in the coming decades. It’s possible the decisionmakers didn’t fully appreciate the effect of radiation on health, although the scientists would have certianly known it’s dangerous. At best they could have claimed ignorance of the exact long-term effects, and I’m not even sure about that.

Also consider the main landmasses nearby were Korea and China, which the US was also racist against, and which were at the time under Japanese occupation.