[pjc50]

The airlines are losing money by having passengers circumvent the price discrimination.

[mynameisvlad]

As already pointed out, the airlines lose no actual money, but you are right in that they lose the theoretical difference between the discriminated price and the one they paid. That's not a legal debt, though.

[FireBeyond]

Precisely. "Potential revenue" is not a dishonored debt.

[panarky]

The airline didn't lose any money.

The airline was paid as agreed for the itinerary.

The seat on the last leg of the flight will be empty, which actually saves the airline money since it will take less fuel to transport less weight.

And the airline can resell that empty seat to a standby passenger and double-dip if they wish.