Roundtrip ticket half the price of a one way ticket?

[slickpoppa]

Can someone please explain the economics of this to me. I am currently looking for one way ticket from Newark, NJ to DC (Reagan). If I search for a one way ticket leaving January 3rd, the lowest price I can find is $313 on Continental. If I use Expedia's flexible date feature and search for roundtrip ticket leaving January 3rd, and then choose January 6th as my return date, the TOTAL price is $151, again on Continental.

Wouldnt a rational airline, given that they are willing to sell me a ROUNDTRIP ticket for $151, be willing to sell me a one way ticket leaving on January 4th for some price less than $151? Or even $151? But instead they charge me $313? This makes absolutely no sense to me. No wonder so many airlines are going bankrupt.