Because cops wouldn't have the uninhibited discretion to kill 8th graders with air guns in a society that truly cared about kids. They instead would have a reasonably trained and skilled police force that could the tell the difference between a real gun and a fake. Our cops shoot first, and ask questions - or cover up - later.
We can contrast the cold-blooded nature of the national response to the Brownsville murder with the outrage in regards to the Penn State accusations. Anything to do with sex and the pillage of innocence deservedly turns up the American outrage meter, while anything to do with unwarranted violence is ignored. Especially when the violence is meted out from positions of authority within our police state structure.