I know that none of you really "know" me. Although some will after the DFW DO!
But I think I come across as fairly reasonable guy who bases his opinions on the facts as he knows them. So I have to say that the "you cancelled the flight because it didn't have any people on it" theory was one of those things that always floored me when I frequently heard it as an airline manager and line employee, and even now as an airline customer. I know that some of the other patently odd business practices of airlines have led to this suspicion, so I never let it bother me when I had to deal with suspicious customers as airline employee and I don't let it bother me now when I hear it from time to time.
But please let me say that in my 10 year, 3 month and 1 week career at American Airlines and American Eagle, I am not aware of ANY flight that was ever cancelled because of low load factor. I've shut the door on one too many empty or near empty airplanes to think that this is a practice that takes place at AA. It doesn't. If loads are anticipated to be low in advance of a holiday or some other special circumstance (9/11/02 comes to mind), schedules are adjusted in advance.
As AEPilot76 aptly stated, it would cost the airline more to displace and deadhead crews, ferry empty planes, etc., etc. than they'd save if this were practiced by an airline.
I now descend from my soapbox.