What makes the policeman different from a meter reader, letter carrier, or delivery driver, in the same situation? Other than that he's allowed to carry a gun?
Carrying a gun comes with the expectation of responsible use. Shooting a dog at first sound of a bark is not a responsible action of any kind.
-PJ
Your comment does not alter what I wrote. In practice the utilities, Post Office, or UPS guys can walk away from a menace, the law officer may decide otherwise. His job description allows him more latitude than the others in pursuing his duties.