What is so special about one way flights that almost guarantees a secondary screening?

Does the TSA honestly think that a serious terrorist will purchase a one way ticket (because they know they won’t need the return leg)? It is such common knowledge now that one way tickets get you SSSS’d that anyone short of a complete moron terrorist would know to buy only round trip tickets. Besides, round trip tickets are usually cheaper than one way and would save terrorist money.

I had a problem about six months ago where I was using a CO code share flight with NW all booked on CO.com. The outgoing flight was NW, but the returning flight was CO. Even though it was all booked on one ternary, all under CO flight numbers, using the CO website, the airline computers looked at the NW flight as one way. Every week, for four weeks, on Monday morning I would show up at the airport and get the old SSSS. That went away once CO introduced a new flight and I was able to take CO metal on the outgoing and returning flight.

So I ask the question again…. What is so special about one way tickets that makes them extraordinarily dangerous to society that they deserve such extra scrutiny? What about one way rentals of U-Haul or Rider trucks (like the one used in the Federal Office bombing)? What about one way car rentals. Surely someone renting a car and not returning to their original rental location must be up to no good, just like those buying a one way airline ticket. They must be planning some kind of suicide attack, driving cars at full speed into the pumps of a gas station.