Of course in the earlier wars (WWII, Korea, Vietnam) the basic was necessarily shorter and less intense so as to have a quick infusion of new troops to send over and replace those lost. The soldiers were often very young and many falsified their ages to serve. The college bound tended to avoid service which skews the results of any theory based on data. In my studies, when children are adequately trained they can often react in the manner which befit their training. It does take more than just a talk or two. My daughter received martial arts defense training and when someone tried to approach her in our yard, she did just what she was taught. She was 9 then. She is 13 now and I still watch her like a hawk but I know she would not freeze up. We don't know what sort of preparation these girls had. The area, like some have said was one where you have an expectation of safety, the small town "everyone knows each other" safe place to live. I think that makes it easier for an unsavoury person to commit such a heinous deed.
You are correct in saying society tells children to use their words and not hit. But in cases where there is danger, some of us instil the idea that it is ok to defend yourself. My father NEVER used corporal punishment on any of us. But I was taught self-defense. In basic training it was reinforced to fight back. In one of my duty stations I role played OPFOR and gave new special forces soldiers a "lesson" in what to do if captured (We were VERY thorough). Now I teach my daughter. If she is ever a victim, she will fight back and she has even assured me she will leave evidence (bite marks, etc.). Being a peaceful person is great, though Sadly these days it seems kids need to learn how to survive or at least take a captor down with them. I wish the world were not so scary...and that these little girls were safe at home, complaining about summer being over.