It isn't just customary here, it is customary pretty much everywhere I've ever driven that if someone approaches you driving faster than you, you should move over and let them pass. This remains true if you are going under the speed limit and they are driving normally, or if you are going at the speed limit or even a little over and they are just going faster.
This is particularly true for the "passing lane" which again, is not only what that left lane is here, but in many parts of the world.
If everyone obeyed that common courtesy there would be far less cause for the bad drivers to pass on the right.

The very fast drivers (you know...the guy in the Landcruiser) do need to learn a little courtesy too. Sometimes they approach the rear of a vehicle in the left lane much quicker than that driver anticipated and there is not yet a place for them to move over (as they are passing a slower moving car as well). The Landcruisers need to learn that just a little patience will get everyone there safely. However, as soon as there is a gap, the slower car should move over to the right lane.