Look, the only reason hidden pricing exists is because the market is not working properly. Supply and demand are not being balanced appropriately either through regulation/collusion/monopoly. United has pricing power to ORD (and many other destinations) that does not correspond to the actual demand to fly to ORD -- meaning that the system is not operating as a real market. I think one could make a moral case that this alone justifies others to ignore their rules and that UA is the one being immoral and enforcing it through a set of CoCs that are creating in collusion with the other airlines, which I presume all restrict hidden city pricing.
If we want to live in a capitalistic society we need to have markets that are free. Customers exposing the lack of free markets are not immoral they are exerting a balancing force that will make carriers charge no more for service to a hub than service through the hub. I think it is fairly simple.