Well, they did sell the right number of seats, like I said it's rare that a plane will be completely full though. In that particular case, I would think the right thing to do is to ask the people who took up his seat to take the next flight and this is coming from a larger person. Luckily someone else stepped in and offered a solution, but it would be what I would expect in a case like that.
Really, I have yet to experience a flight where every single seat was full, but I guess I should have said that Jetblue does no intentionally oversell their planes like Southwest and so many others do.