This was not a regular airline ticket. This was an airline ticket purchased by Disney from Delta on behalf of the cruisers, under the contract between Disney and Delta. Disney didn't pick the flight times, Delta did. What the contract covers is Delta's obligation to get the cruisers to the destination in time to get on the ship before it sails. If it cannot do that, it is Delta's obligation to get them to the next port that they may legally embark the ship (in this case it was Boston because the ship was not returning to Port Canaveral) and to accommodate them while awaiting the ship.
So Delta was not stepping up as the hero, it was fulfilling it's obligation to Disney under their contract.
I've dealt with enough Cruise lines Air/Sea departments to know you roll the dice when you book their air on the embarkation date. The airline ticket is pricy but if you select it for whatever reason, it's worth the deviation fee to come in early.