Why can't I jump off before the final destination on my ticket?

Q. I want to fly from Burlington VT to Philadelphia. Nonstop fares are $800 RT. Burlington to Newark is $280 RT nonstop. Burlington to Newark with a connection in Philadelphia is $200 RT. So why can't I just get off in Philadelphia?

A: Because if you buy a round-trip ticket, they will cancel your return flights, should you get off in Philadelphia. If the ticket is sold one way, and you're only going one-way, then yes, you can theoretically do this, but the airlines frown on this practice because it depletes their revenue. What can they do about it? Nothing, legally, but they might kick you out of their frequent flyer program or take similar actions within their powers. And in fact, that route is sometimes as low as $200 RT anyway, if you wait for an unadvertised sale. And Burlington to Baltimore is usually around $100 RT, so you might consider flying into BWI and taking Amtrak up to Philadelphia.