(2/6)In the old days, might made right. If you were an oppressed person, you still had to be 'nice' to your social betters, but they were under no such obligation. They could mostly harass and assault you in any way they pleased without fear of repercussions.
But, as hard as it can be to believe sometimes, society has evolved. Society demands that we appear to be civilized—that we be nice, polite, friendly, etc. We have come to like a certain amount of stability in our societies, and we have learned that open displays of aggression lead to an awful lot of fighting and revenge and war, things which disrupt the peace of social stability.
So society demands that even powerful people, even oppressors, maintain the appearance of being nice. (In the case of police and military, to at least not look like blood-thirsty maniacs.) No more Old Testament badassery—that'll get you shunned from your community; we're supposed to be New Testament, love and peace, above wanton violence and barbarism.