Im a musician and have a violin worth around 50,000 dollars. I have a case that's compact specifically for traveling. One time a GA was going to not let me board the plane with my instrument. I almost went home as checking it is obviously not an option. At the last moment she changed her mind and let me bring it on board, but I was pretty close to missing my trip. It's a very frustrating situation especially since rules can vary, depending upon which person you get.
It sounds like, from the article, this person tried to purchase an extra seat and was denied. If he was denied and forced to check the guitar, I definitely think he's owed compensation and I hope he gets it.