Because it's more acceptable to kill a foreigner than a fellow countrymen?
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I think that it's because most people don't have any experience with the military other than the little bit that the military wants us to see. The police, on the other hand, are common to see. Most of our involvement with the police involves dealing with tickets, and getting arrested, and other such irritations. It's way easier to revere the people who "defend your freedom" than the people who write speeding tickets.