Originally Posted by Ninety5rpm

it has everything to do with the motorist's behavior in reaction to the two positions.

A motorist who is approaching an edge cyclist typically does not alter speed or position regardless of whether the edge cyclist is seen. So an edge cyclist with a mirror can't discern between the two motorists - both are behaving the same in terms of speed and trajectory even though one is alert and the other is inattentive and about to drift into the cyclist.

A motorist who is approaching a cyclist in the lane most does alter speed or position (if not both) in reaction to the presence of the cyclist up ahead in his path. So this cyclist with a mirror CAN discern between the two motorists -should one be so inattentive as to not notice a cyclist directly in his path he will be be blatantly obvious (neither slowing nor changing his trajectory). Or, in this case, where the motorist want's to teach the cyclist a lesson, his trajectory will be towards a buzz pass or actual contact rather than one headed for safe pass.

Spend some time with a mirror and experimenting with lane positioning and you'll see what I mean.