Please quote your source for that. I would love to see in writing that the airlines are contractually obligated to get you to your destination or give you a refund if you are on a single TICKET (single "reservation" is irrelevant, as per my discussion above).
It’s in the contract of carriage, IATA regulations and .it is also a very basic part of U.S. common law. In layman’s terms "you get what you pay for." You pay the platting carrier to get you from point A to point B. If they can't deliver they can't keep your money. If you notice when you buy a multi-airline ticket, regardless of whether you use the airline website or a travel agent your credit card statement will have a charge from only one airline. This is the platting carrier. Your contract is with them. They can't keep your money if they can’t give you the services you paid for (unless bankruptcy is involved) any more than Amazon can keep your money if they can’t send you an item you ordered. Luckily for us the airlines would rather give you alternate transportation then your money back.