Your friend the cop has poor math skills. Let's say you are riding at 20 kph (13 mph). A car doing 50 kph passes you going the same direction. You've effectively reduced the speed differential between you and the car to only 30 kph. If you were riding 20 kph in the opposite direction, you'd increase the speed differential to 70 kph, giving the motorist far less time to see you and react to your presence.
The usual reason give for salmoning is that you can jump off the road if an oncoming car is too close, but that assumes that you have somewhere to jump to and that you'll be able to react in time.