Hmm... I clearly don't understand the terminology used by the military, but I do think of "killed in action" as death attributable to active combat. I believe that many deaths of soldiers serving on or near the front lines (particularly prior to the 20th century), were due to things like disease and malnutrition, not military action. The family may only learn that their son or brother died "on the Russian front," not whether combat or cholera caused his death, so it seems like an exaggeration to assert that he was "killed in action."