If the Rental Brokers try to do this they will go out of business quickly. Cancelled reservations is one thing, but someone coming to the internet to post that their reservation was cancelled through no fault of the renter and DVC Rental Store or David's refuses to refund? Yeah, that would be enough to drive their business under. Nobody would use them. Doesn't matter what they write in the contract.
I seem to remember a few months ago, last year sometime, a few renter's reservations were cancelled by Disney. I don't remember what happened with that situation however. It didn't seem to have any ripples in the DVC rental industry.