I don't think rationality is a good way of looking at it. Rationality is defined over some goal, and there's no obvious goal here. The better way of looking at it is to say that the military teaches them to treat their own life as less valuable. Lives have no inherent value or disvalue, beyond what we apply to them, so this is neither rational or irrational. But the effect is that they are more willing to, as you say, "log off".