From my reading, the volunteers were not actually informed of the true danger they faced. In multiple instances either no warning was given or the truth hidden, saying that the background levels were harmless when in fact they simply had no equipment that could measure high enough.

In addition, I'm skeptical that the biorobots and liquidators were true volunteers. If the Soviet government comes to your house and hands you three sets of clothing saying "we need volunteers", you didn't really have a choice.