Because one of the key challenges of law enforcement is securing community cooperation. Law enforcement cannot be effective when hey are seen by the community as a hostile gang of thugs, and politeness is just a name for adherence to social standards than signal that you are part of the community and not a hostile outsider.
Sometimes, yes. Other times, such as when a robbery is in progress, or when an accident has just occurred and you've arrived on the scene, polite and kind aren't very important.
Fair and law-abiding become much more important in these situations. Once the danger has passed, kind and polite can be important again.
