If you think about it it makes complete sense. If he is not carrying any more weapons then he is no longer a threat - killing him is entirely unnecessary and there's no reason to call it anything but murder. If he is carrying weapons, or you believe that he is still going to be a threat towards you or others then you can use controlled force to stop that happening. That's what the UK ROE is anyway, not so sure about OOA.
What a lot of people seem to want is for the ROE to be more like vengeance - "He shot at me so I get to shoot him!" - which really, really wouldn't work.