The answer isn't as obvious as you think, and if there is no effort made by LEOs to differentiate themselves from burglars except by breaking down the door and yelling "Police", then they will get shot at by those whose homes are being invaded.

If you have a warrant, treat me like I'm innocent -- because I am! -- and then you'll get what you need without further ado. Break into my home, however, and run the risk of being treated as a common thief.