score:2
Economics, it's likely that demand in one direction is higher, and as such the airlines can charge more. Or competition is lower in one direction, so they can make profits. Or landing / airport taxes might be higher, and they put it on that. This happens all over the world, and is part of travel hacking, trying to work out if you can get it cheaper ;)
We have similar questions about other routes, so I'll link to those for completeness:
However the airline still needs to get back to pick up the pricey passengers, so it's in their interest to fill the plane, even if it's at a lower rate.
As for discarding tickets, this happens, although airlines aren't always pleased. I've seen a return ticket be cheaper than a one-way regularly. Fortunately, we have a question on this too: Is leaving airport at a stop before the final destination illegal?