Yes it is, he paid for the seat and has every right to be there. If he was unruly, sure. But he wasn’t - he held up his end of the bargain, and had every right to expect the airline to hold up their end.
Except that paying for a seat doesn't actually give you the right to be there. I'm sure none of us have fully read the contract of carriage. But you can certainly pay for a seat but be denied that seat. It's not great, and I wish customers had more power in this situation to be able to negotiate a better contract, but... that's where we are.
