Interesting question...

The reason why airlines ask for volunteers is that they are required to do so by Department of Transportation regulations.

The DOT does not require that the airline offer any particular amount of compensation, or any compensation at all, to vollunteers but airlines typically do. The DOT DOES require specific compensation amounts in the event of an involluntary denied boarding and that compensation must be paid in cash or check. The amount of required compensation is based on the fare that you paid and the length of the delay that it causes in reaching your final destination. Caps are $200 or $400 depending on the circumstances.

What most airlines will do is offer volunteers a higher amount of compensation than what the DOT rules would require for an involluntary bump but the compensation will be in the form of a voucher or free ticket. The incentive to the airline is that it costs them much less than to face value of the voucher to provide the "free" transportation when the voucher is redeemed so they may end up spending only $20 in actual costs instead of paying $400 in cash to an IDB. The incentive to the vollunteer is that they get a higher face value which is worth it's full face value if the passenger was/is going to fly again anyway.

The DOT has a pamphlet called "Fly Rights" which covers this, as well as other travel rights issues, in detail. You can read Fly Rights at the following URL:

http://www.pueblo.gsa.gov/cic_text/t...s/flyrghts.htm