It's actually the federal government that has determined what is appropriate for forced bumping from a flight.
* No compensation for less than one hour delay.
* 200% reward in cash for one to two hours delay.
* 400% reward in cash for two or more hours delay.
* This is all in addition to retaining your original ticket, which the airlines must continue to honor with a new flight. Or you can request a refund, though I'm unsure how that interacts with the rewards.
The vouchers are the airlines attempts to get volunteers for cheaper than the federally-mandated compensation.
https://www.transportation.gov/airconsumer/fly-rights#Overbo...