Originally Posted by
sethb
"exercise unauthorized control", yes.
"intent to deprive the airline" no. The passenger took an empty seat, which had no use or value for the remainder of that flight.
It still has value. You're confusing the fact that no one not on the plane can't buy the seat, with the seat not being sellable. Just because they can't sell it to anyone other then someone on the plane, doesn't mean the value is gone. If there's no value, then how do some airlines offer upgrades in flight? As noted earlier, DL CoC even details a pricing scheme for such upgrade.