That’s just it. They are never off duty within tehir jurisdiction, so they have an obligation to use lethal force when appropriate. The difference is the way in which ordinary citizens were once upon a time obligated to be armed as well and under somewhat limited obligations to use lethal force in appropriate circumstances in defense of other citizens. This civilian responsibility for public safety has been deprecated.
Laws are easy to change. If they aren’t on duty (scheduled and getting paid) then they can leave their gun at home or at the office.
Just like they want us to do in so many places.