Sorry, but I've done this too. I bought the product, it's up to me to use it fully or partially. If I get off and don't take the next flight, then cancel that portion and give it to someone else (airlines already do both these things).
DTW-EWR was over $300 one way. DTW-EWR-AVP was $75 one way. Guess who's gonna buy the latter and ditch in EWR? The only risk that day is if the airline might re-route you to get to that final city in which case you're screwed. For AVP, at the time the only flight on this airline was from EWR so I knew EWR would be safe. From a pax perspective, buy the ticket one way and don't check in a bag.
A lawsuit is ridiculous. How does the airline know that I wasn't really sick once I was in EWR, and decided I shouldn't fly the next leg? Maybe I thought I'd call it a day and just drive to AVP? That's my business. Lets be honest, the skiplagged guy was genius and the airlines are POed at him.