The motorist is 100% at fault since he was the one driving on the wrong side of the road. All of us have doubtless been in the same situation as those riders. Some of us didn't get hit because we were going a bit slower, some of us are just a bit quicker (probably because we expect motorists to behave badly), some of us have gotten whacked and some of us fit more than one category.
My favorite short loop to ride when I don't have a lot of time involves similarly narrow descents (for CliffordK: Fox Hollow and McBeth). I encounter cars on the wrong side all the time and I just deal with them. In fact, because poor driving is so common hereabouts, I expect every car to be on the wrong side of the road when the roadway curves to my right and I'm rarely disappointed.