They don't need to. They've got the revenue for that seat for that show, and they don't have to provide any alternative.

If an airline sells "travel or bust" tickets (no changes or refunds, before or after departure), they don't need to either. They've got the guaranteed revenue for the seat. They don't have to care about it. It makes life simpler (and cheaper) for them. And the combination of "travel or bust" and non-transferability means that people really tend to travel as booked, which is also simpler and cheaper for the airline.

Anyway, I was only giving an answer as to why I don't think there's a legal problem involved. There are obviously more complex business issues.