Every action involves some risk of developing a mortal condition, and being in the military, particularly, involves notable risk of being assigned to tasks that have elevated risks of developing a mortal condition.
You're being asked to fly into a radioactive mushroom cloud to test if you get harmed or not.
And you're probably not even being asked but ordered, you don't have too much choice.
Oh, he died? Well, let's test again but 20 minutes later instead of 10
The link between radiation and cancer was established in the '20s, and certainly well-known (if poorly understood) by the time atomic weapons were developed. For instance, Muller got the Nobel prize for this work in 1947.
https://en.wikipedia.org/wiki/Radiation-induced_cancer#Histo...
Certainly, safety rules weren't as conservative and stringent as they are today, but it's silly to think US soldiers were routinely and openly used as guinea pigs.
This is pretty important since you can both evaluate your weapon, and also to calibrate your instruments to use when collecting downwind of Soviet tests. Thus you can evaluate their weapons too.