How exactly is the seat unsold in this case? It was sold, it is simply unoccupied.
I agree. If anything the airline made *more* money as it didn't have to expend the fuel to get the passenger from Chicago to LA.
Not only that, but if the seat is unoccupied when they're ready to close the doors, they can let a passenger on if the flight is overbooked, saving the airline both a seat on the next flight, and whatever compensation they had to offer the passenger who would have missed his flight because it was overbooked.