alt Hacker NewsI don't see why they would _ever_ prefer an unsold seat unless:
* they are trying to artificially inflate the price
* they could send cargo that is, per pound, more lucrative than the cost of a ticket
It's not like the weight of a person is going to cost an extra $50 in jet fuel
There's a surprisingly high price floor on airline tickets. Like $30 (one way) in fees to the government and the departure airport. Plus the incremental fuel [0] and any complimentary consumables, etc.
Not much of a factor on more expensive fares, but for short flights where a $50-100 fare is realistic.
[0]: even with today's oil prices, the extra fuel is only $20-30 for a transcontinental US flight. negligible for short flights.