Just a few questions:
Was anyone sure they saw a bear, or was it merely a dark large dark object rustling around in the murk of the evening? Could the riders have been mistaken for a bear? Is it likely or natural for a bear to be on the move at that time of day?
Lastly is it common to get mugged on the trail? It all sound a bit paranoid to me. The kind of event that rarely happens, but when it does it scares people into being overly cautious - a bit like stories of man-eating bears.
Thankfully I have no experience of bears or outlaws, but your reaction seems perfectly natural to me.