Spirit's situation is even more clear-cut than your example. They are selling a perishable commodity. That's what so many people are missing. You are purchasing the rights to a seat on a specific flight at a specific time. When you purchase that ticket, the airline can't sell it to someone else (beyond a small percentage of intentional overbooking). If you don't show up for the flight, it takes the same amount of gas to move that seat, which is a loss if a refund is given and nobody fills the seat.
You're not purchasing the right to travel anywhere, anytime; you are buying something that only exists for a short period of time, and if you don't use it, it can't be sold to someone else. It's like trying to return a gallon of milk after it goes bad, saying that your doctor advised you to stop drinking milk. Should the store give you a refund, now that you prevented it from being sold to someone else?
