I fully agree with you. There is no doubt that the vast majority of people who witness an event such as the one you describe—body parts strewn about that they have to pick up—will experience stress. However, acute stress response is not just stress. It includes symptoms such as flashbacks and nightmares. You also have to have experienced dissociation when the event occurred, have problems concentrating, have symptoms such as hypervigilance, and so on. Not everyone presents with these symptoms. It's more serious than the simple stress response that most people have. It's a little different.
On March 22nd, 2007. See this statement in context.