i disagree the refund should be 100%. We already have that under current consumer protection laws. But then you don't also get to fly. It right that the airline still gets paid *something* if they transport you.
They usually get much more than 'something'. In the over-booking situation, they got to sell your seat to someone they had no right to sell it to. Their gamble failed. They received the price of two business class seats, when they only had one available. They are already profiting from the situation, and an ongoing business strategy that demonstrably and knowingly puts the right of passengers to receive the service they paid for in regular jeopardy, in the pursuit of a business gamble. If they are to be allowed to make that gamble with passengers' rights, they must be required to compensate fully when it fails and a passenger is effectively forced to accept a service they never intended to purchase.
The downgraded passenger is over a barrel in often having no choice but to fly. So it's not a normal consumer-company transaction; it's unique, and so may require unique terms. One might frame it similarly to situations where special protections are afforded to those who are particularly vulnerable to exploitation. The 100% refund (or more in some cases I suggested), simply claws back some of the profit obtained by questionable means and gives it back to the 'victim'.
I think the policy needs to be firm in rejecting the notion that a downgraded class of travel is in any way fulfilling the terms of the original contract between the airline and the passenger. The current practice redefines that contract such that a consumer can be effectively charged for a service they did not intend to 'purchase'. Are there any other precedents in consumer law where that can occur legally ?