For walk in sales, that is probably correct. A price is mis-marked, a few customers get the deal, yes you have happy customers. However it is not clear that online customers have such loyalties. They will tend to go where the low prices are, as there is little opportunity cost for doing so. That is why Amazon has what much a loss leading Amazon Prime program. To keep customers coming back not just for low prices, but other perks. Same thing for airlines.
So no, the rules for online are not to fullfill orders that have clearly incorrect prices. If I go into a big grocery store like Krogers, and some disgruntled employee has put a a 50 dollar bottle of wine on sale at $10, they are not going to sell it to me for $10 when it rings up for 50. There is a secondary check there for price, the human element. Likewise, if a computer glitch, maybe put in by a disgruntled employee, allows me to check out for half price, then this is an admitted grey area. My payment has been accepted.
I would say, however, that until a product is formally charged to a customers card, which often happens as it is shipped, and maybe even until it is delivered there the retailer has an opportunity to cancel the order. Possession is, of course, paramount. This is why I would say one the product is delivered the price must be honored. This is a grey area as well, and we have seen cases where retailers have demanded delivered products back, but this to me is clearly bad manners.
So why is Delta honoring the price? I think it is because of delivered product. When I buy a ticket, my card is charged, and I immediately get a confirmation that I am guaranteed a seat on that flight. If something happens and I do not get a seat on the flight, I at minimum am sure to get a seat on a similar flight, often with financial compensation above and beyond that seat. Also, unlike most small retailers, the airlines have algorithms that continuously adjust the price of seats to maximize the total revenue on each flight. Therefore it is harder for airline to use the 'disgruntled employee' excuse.