Both soldiers and police carry weapons. Because most people here in the West only see representatives of the state carrying weapons in the context of law enforcement, they assume that that's the only reason such representatives can carry them. In the police officer's case, the sidearm is there as the last lethal resort to be used in order to uphold the law and preserve order.
A soldier's job is different. Her job is to die if needed by her country, but mostly to see to it that others die for threatening its interests. Using her sidearm, she may wound people, but that is in order to stress the logistical resources of their side, rather than out of any reluctance to kill. Incidentally, sidearms have not been important sources of battlefield deaths since the American Civil War, a fact often ignored by gun nuts.
Anyhoo, this failure of understanding leads people to expect soldiers to behave like police officers, which would mean that they could not do their job. Let's not slack off on holding soldiers' behavior to reasonable expectations, but at the same time, we need to understand what "reasonable" means in the context of a battlefield.