As the OP said, because the man was already boarded that is the issue. For people to defend the airline, frankly, you're nuts.
If this happened in the terminal, that is one thing, but because he was already seated, UA should have bit the bullet and figured out a different way.
No the CEO is doubling down on blaming the passenger, when his focus should be on the employees. This is what is so upsetting to a lot of people.