Police officers in America overreact everyday, for the same reason dogs lick their nuts — because they can.
That's why human beings are evolved to overreact to certain kind of threats.
We should not be surprised when they occur, and most importantly we should not overreact if one is eventually successful.
overreact o·ver·re·act (ō'vər-rē-ākt')
v. o·ver·re·act·ed, o·ver·re·act·ing, o·ver·re·acts
To react with unnecessary or inappropriate force, emotional display, or violence.