Here's a similar reference a little closer to my home -- Joint Federal Travel Regs, Nov 07, Chap 1, Part B, Para U1200:

"B. Seat Relinquishing

1. Voluntary. A traveler may keep payments from a carrier for voluntarily vacating a transportation seat. However, no additional expenses (per diem or miscellaneous reimbursable) may be paid as a result of the traveler’s delay. Additional travel expenses incurred as a result of voluntarily giving up a seat are the traveler’s financial responsibility.

2. Involuntarily. If a traveler is involuntarily denied boarding on flight, compensation for the denied seat belongs to the Government (59 Comp. Gen. 203 (1980))."

I'll be damned, I did not know this. Not sure where the logic is in this either...