That is good input.
I guess my question would be, "How would Disney handle this if the OP's BIL was just a cash (credit card paying) vacationer?".
Renters (or DVC members) need to provide photo ID, address, etc. upon check in. So, credit card or not, the room occupant is identified.
So let's say, hypothetically, he booked a cash trip.
He gives them his credit card, they somehow lose the info, he checks out and asks why his bill is $0, they smile, and he leaves.
So, does Disney just eat that charge? Does the resort try and track him down?
Seems funny to me that if mistakes are made, and if it is a DVC member's rental that it becomes the member's problem.
I am not disputing the conclusion that the member is liable for charges / damages by renters, just that there is a double standard being exercised here.
I can see the front desk supervisor saying "OMG, we didn't collect from that guy! Good thing we have a DVC member to go after to get the funds, otherwise we'd be stuck"