If he was on the plane illegally (and if he surrendered his seat/ticket that would be the case if he refused to leave- the fact he once held a valid ticket would mean nothing once he accepted compensation [if the NBC report was accurate]) then he was going to be removed from the plane one way or the other- since 9/11 and the enactment of new TSA guidelines, airline security has rights within the aircraft and airport that go pretty far as far as conflict resolution goes.
In the US, it's a bad idea to to play the 'If you want me to comply, make me' game with airport security, because they're going to make you.
Again, if the report I posted is correct, then they were 'justified' in a legal sense- I'm not going to argue that is seemed way over-the-top and it certainly seems like it should have been handled differently. Without all the facts, all we have are assumptions at this point.