If im understanding the problem right, it is fair to complain about the U taking your money, not providing the spot you paid for, and not refunding your money, even if the reason for the spot unavailability is reasonable.
Everyone knows airlines overbooks their flights. That means sometimes someone gets bumped to a later flight. The airlines dont get to say "sorry, we're full, you need to go find another ticket to buy if you still want to fly, but we're keeping your money for the seat on the plane we did not provide you."