I agree that this is the most reasonable explanation, but it also places some of the blame at Expedia's feet.

I book through Travelocity all the time. The Travelocity map color codes preferred seats differently and, upon selecting a preferred seat, warns you that those seats are only for frequent flyers.

If Expedia isn't doing that, then they're doing their customers a major diservice.

And if they are and this guy was trying to nab a window/aisle in the back of the plane (i.e. not preferred seating) then someone needs to write him and tell him about DL IT. Tell him it's a rite of passage :-/