I don't know how you can determine a threat is over by someone just running away. What if they turn around and shoot you. So your hands are tied until they turn around and by the time you react you may be dead because you waited. To me there is no reassurance in seeing a thugs back side. And he/she could turn around at anytime and finish the threat. It's a gray area for me. And a decision I never want to make.
Is the BG actually 'fleeing', or is it a 'tactic', a subterfuge to flank the defender and attack, a feign to re-attack, or to perhaps run back to the BG's car and re-load or grab a better weapon (say, a shotgun rather then a crowbar, etc.)?
Yup. I had a chrome saturday night special shoved in my face as I was chasing a yout out the door of a sporting goods store I worked at as a teen. we both cleared the front doors and right after I made the hard left onto the sidewalk he turned his torso and pointed the gun at me and yelled to get back. Of course I did...
So yes, even running away, they can still be a threat.