Without getting into all the details (and I'll concede that I may not know all of them) one thing strikes me as the most salient point: if I pay for something (presuming that the something in question is not itself immoral) it is morally mine. I have moral power, moral control over it.
The arguments about the logistics of airline overbooking are bunk. I pay for a seat from Chicago to Louisville on a certain date at a certain time on a certain airline, that seat belongs to me. I control it. I own it for that period of time. To make me give it up involuntarily is a moral evil.
I sell drain snakes. If a man paid me up front for one with a promised delivery date, I must honor that commitment. I cannot unilaterally sell it to someone else or use it for some other purpose (especially selfish purposes) outside of extreme and compelling circumstance. Period. Simply to get your employees to a destination is not a good enough reason to delay others. No others, again outside of compelling reasons.
That's it in a nutshell.