Hotels do the same thing. The biggest problem is when their match clashes with modern culture/events.
Example: nerd conventions.
Flights to Boston, and hotels IN Boston, for the first PAX East were basically completely booked solid. The expected rate of no-shows and cancellations was x, but the actual rate for PAX was around 0%. Thus, they fucked over a lot of people trying to get to PAX (voucher doesn't help you if you miss the thing you were flying for in the first place), and they fucked over a lot of people who made it to PAX (sorry, there aren't hotels).
Nowadays, most of the people flying regionally don't have a lot of money or vacation time, so their travel is tightly scripted. They can't afford to reschedule. Further, they don't fly enough to be able to use a voucher, so it's basically worthless to them.