Yes, you did say it: "Sell the second leg to someone who needs it. Planes are so full these days, someone would purchase that ticket." Well, they can't sell it to someone else because the airline doesn't know until the last minute that the person who wants to get off in the middle is not going to show up for that second leg.
No, in this scenario the airline doesn't get all of its money. Somebody wanted to fly point A to B but paid the lower price reserved for people flying from point A to C. The airline loses the difference between those two fares.
As I said earlier in the thread, if the airlines charged separate fares for each leg, people who have to fly on connecting flights to get somewhere they actually want to go, which is a large number, would actually end up paying more overall. Seriously. In this scenario, the kid was ticketed on AA from Gainesville via Charlotte to New York. Go online and price an AA ticket from Gainesville to Charlotte. Then price Charlotte to New York. Add the price of Gainesville to Charlotte with the price of Charlotte to New York. That added fare will be more than the price of Gainesville to New York with a connection in Charlotte. If you were someone who actually wanted to fly from Gainesville to New York, tell me that's really how you would want the airfare to be priced.