Yea totally agreed and I pointed this out. The rules from the Dept. of Transportation are for consumer protection and not for airline protection. But you are correct that employees have now twisted this to mean that they are "legally" bound to pay a certain amount and no more, which is horse hockey.
That's complete BS. It may be a United rule, but the law only defines how much they are required to compensate people who are denied boarding. When asking passengers to voluntarily not get on the flight, they are free to negotiate any deal that is acceptable to the two parties.
That the airline doesn't have to offer more before they execute the involuntary denial of boarding somehow has gotten twisted into the fact that they can't. That's simply not true, they just chose the cheapest way out.