They probably argued that the test was more like surveying defenses than an experiment. i.e. "we need to try it because we need to know if we should be concerned about the Soviets doing it with seriously harmful bacteria."
Additionally, when you do anything involving a group that large there's bound to be an few outliers. Winning the lottery, getting bitten by a shark and dying from complications caused by normally harmless bacteria are all incredibly uncommon but they still happen.