The U.S. law is quite customer friendly. To be considered a VDB, the airline must 1) solicit volunteers, and 2) you must agree to be a volunteer. If the airline picks you for whatever reason (check-in time, no seat assignment, or just plain bad luck), and you do not have the option to board the aircraft, you are not a volunteer, and it is considered an IDB, making the customer entitled to cash compensation. Further, if the airline does not solicit volunteers for compensation, then the airline is breaking U.S. law and can be fined.

If you're an IDB, obviously the airline wants you to take the voucher, and may be reluctant to give you cash. They often offer more in voucher than in cash. If they refuse to give you cash, you can always take the voucher, file a complaint against the carrier with the FAA, who may rule in your favor, and order the carrier to give you the cash you're entitled to. They will also likely not be able to reclaim the voucher, as that was a "gift" to you.

Perhaps it's the behavior of carriers like Qantas which is why the law is written the way it is.