I refer to the fact that there is a regulation that allows a compensation cap and if there are no takers, call the cops. Remove that regulation and United would have to figure out a different way - like offer more money or face the threat of a suit. (Remember, the way things currently stand, United look like massive dicks, but they did not break the law.)
The cap applies to involuntary denied boarding. The airline can offer whatever they want for volunteers. That does, of course, encourage a certain pattern.
Remove that regulation and united offers you $0.01 and a pat on the head for not letting you board because they overbooked.