I love analogies. I use them all the time. But many of the analogies presented to the OP are mixing apples with apple picking. Best Buy, Barnes & Nobles, Ford, and grocers (for the most part) sell products. Delta and other airlines (for the most part) sell a service - travel from point A to point B.
OP, you could buy a fully changeable fare and not incur the $250 fee. All airlines are more than willing to sell you flexible tickets. But those fares are much more expensive. By offering you a cheaper fare with restrictions you are making a commitment to fly the flight you chose and Delta (and their ilk) will plan on you being there for the flight. If they raised fares they wouldn't come back to you and ask for more money. In fact, I suspect you (and others) would have many many threads griping if airlines did this. So for them to offer you a refund on the fare difference for a price drop is unreasonable.
While the Southwest reference is quite accurate you should know that WN does offer fully refundable fares like Delta. But most people buy the non-refundable fares. While they will not charge you a fee if your plans change they will only let you apply that money to another ticket. So if your plans change and there are no cheap fares for those flights WN expects you to pony up the difference in price. It is great that they don't charge you a change fee but they have a very different biz model than AA, US, UA, CO, and DL (as well as others). In fact, WN is more the exception than the rule when it comes to change fees. By all means use WN to avoid change fees. They have a huge following of loyal customers that cite this very policy as their main reason for flying them.
But your situation was not a change in itinerary just a price drop. Even when WN offers ding fares they don't go back to everyone who bought a higher fare for the same itin and refund the difference.
When did you originally make your purchase? DL will give you till midnight the following day to get a full refund on any ticket purchased with no change fee. I have canceled a DL ticket and re-booked the last three days in a row because an August itin dropped each day.