if they feel their life is threatened, they can use deadly force.
If they could pit at will, there would be lawsuits from people saying they weren't necessary, etc. You can't argue the use of deadly force if the perp is trying to kill you though.
Honestly, I think it's a load of shit that the cops have as many rules as they do. It puts the letter of the law on the side of the criminals and gives them too much freedom.