Having been in the Army (though never in combat) and been a LEO, I have been alarmed for some time about the way military thinking and military training have become part of LE trainng. There is a lot of difference and some of today's police, trained by military veterans, seem not to understand that.
In combat (see above), a soldier would rarely worry about what is behind his target. His mission is to take an area, to control ground. In many cases, that means to kill the enemy (not to wound him, or shoot off a hangnail, to kill him). That also means that in many/most cases, the soldier cannot and does not care what is behind the enemy or what happens to the bullets that miss or penetrate the enemy's body. He will use full auto fire or fire from a belt-fed MG to lay down suppressive fire. He will do whatever it takes to accomplish the mission.
But a police officer can't do that, or at least shouldn't. Every time there is an emergency, I see TV coverage of what seems like hundreds of police with AR-15 selective fire weapons, high capacity shotguns, submachine guns, and even grenade launchers. I simply don't believe that the police carrying those weapons are fully trained and skilled in their use. In a combat zone, it doesn't matter who gets killed - the military calls it collateral damage, sees it as unavoidable (which it probably is), and the fault of someone who should not have been there in the first place.
But what happens when a hundred cops, armed with SMGs and AR-15's and grenades charge into a school to arrest a 10-year old who pointed his finger at another kid and said "pow". A nice peaceful situation, or a mass slaughter, with cops shooting at other cops and kids caught in the middle?
Yes, we do have crime in the U.S. And some of it is gun crime. But are the police arming to stop a stickup at the corner 7-11 with thousands of rounds of machinegun fire? Or are they buying all that expensive military hardware just to have fun on the range? Or are they planning for the day someone in power says he or she wants to disarm the American people and doesn't care what it takes to do it?