The compensation rules for volunteering and denied boarding is different and since they were seeking volunteers to disembark the aircrafts, that would means that passengers who have taken the bait of a $500 qualified for the voucher instead of cash.

To my knowledge, those passengers who were FORCED off are entitled to cash compensation and assistance with rebooking for next available flights, regardless of carrier.....but depending on how that was handled, it's a gray area who should and who shouldn't get this cash option.

Jiburi