If you gave someone a map and asked them which way is North and which is South, I doubt that many people would hesitate before answering correctly.
But ask which is East and which is West, and I expect that quite a few would go through the "Never Eat Shredded Wheat" routine to check!!!
As to why you observe this in Heli pilots rather than fixed wing, it's probably much more obvious as they approach low level to the boundary from the "wrong" direction.
A fixed wing pilot making the same mistake will probably join the circuit anyway, and pop up either Downwind or on Final where you expected them?
RC