Airlines pay a nominal fee to re-accommodate passengers on each other's flights in the event of a disruption. The fee is nowhere near the face value of the ticket, and no one is going to pay a walk-up fare in the absence of an agreement to re-accommodate in the event of a disruption. Airlines use industry discounted rates in cases like this. Sometimes it is a matter of no agreement being in place (as was the case between AA and DL for a few years), and passengers can't be re-accommodated even though there are seats on the flight of another airline.