What you guys are all referring to are excursion tickets, or seat sale tickets. One way tickets are available on excursion tickets but they are generally about 65-75% of the return fare. Hence when travel sometimes is required for a oneway from US to UK, it is better and cheaper to buy a return ticket and not use the return, so to speak.

But if you're all looking at the full economy fare (which some people still do buy), you'll notice that it is exactly HALF of a return fare.