This is something that truly fascinates foreigners who have never seen this practice in their own countries. I think it may go back to the days when people were being encouraged to take leisure trips, plus the fact that one way fares are more likely to be bought by people on working trips (and maybe more able to pay or needing to make the trip with not much choice). The net effect is to suggest that the single is overpriced. The same applies to the traditional airlines, low cost ones having upset the traditionalists.