Is sneaking into a movie theater not stealing either? And even if not, what about the extra fuel that needed to be burned because of his weight?
The movie theater example seems similar to the airplane: as long as there's an empty seat then it seems like stealing isn't quite the right word. Freeloading might be more apt.
So I’m not going to be a Karen.
I applaud him for the creativity.
Plus him breaking the rules has led to improved security for you.