And to manta49, please don't come away with the idea that Americans believe every front-door encounter is a life-threatening event. It isn't. 99.999999 percent (okay, I made that up) of our encounters are entirely benign. But here in the states, we have the right to be ready for that 0.0000001 percent when it isn't.
Plus, we can keep the king of England out of our face.
(An American pop-culture reference, if you don't recognize it.)