There is something about having people walking around in military equipment in a civilian setting that is just un-American as I see it. The police aren't some separate entity, they are citizens, like us. There is something unsettling about it. Something not right.
I don't remember this sort of thing when I was a kid. We had planes in the sky and soldiers--unarmed--in our town, but the idea of sworn law officers moving about in mass with all this amour and iron hanging off of them was just unthinkable to us. A lot of it seems to have arisen from the war on drugs and the tactic of trying to take down big dealers and raiding houses without knocking on the door, etc. The old English Common Law never contemplated a "no-knock" raid--another reason that it isn't just conservatives that should care about the original understanding of the Constitution.
We put together these quasi-military forces and they eventually become part of the landscape. We accept it as normal. The organizations find ways to justify their existence by finding more and more things to do. Gradually we become less citizens and more subjects of the mandarinate.