People who fly out of ATL, or who strongly favor Delta because of their origin city or the benefits of/desire for status, are a captive market. Within reason, they can be milked.

People who can also connect somewhere else, who might fly DL via ATL or another airline via some other city, aren't. That market is more price-sensitive. If DL tries to charge their captive-market ATL fare plus something additional for the connecting flight, those people have options. Many or most of them will vote with their pocketbooks and fly a different airline,

It's about maximizing profit. As long as it's done within the bounds of law, regulations, and ethics, it's not evil. They set the rules. Then we get to play the game, If we can find lower fares by starting and ending our trips in odd places, that's within the rules. Beyond that, there can be grey areas - which would be topics for other threads, many of which already exist.

(This example used DL and ATL, but the logic applies to any airline with a captive hub.)