I don't think there is anything illegal about no compensation as long as it's voluntary. The pax might have thought that an early flight was enough for him to forgo monetary compensation (however, he likely didn't realize he had leverage to ask for more than just an earlier flight!). In VDB, as long as both parties agree to the terms, it's legal.
LAX
And to add: IIRC if you are IDB'ed to O/A and they get you there within 1 hour, there is no IDB compensation due.