I think the man's behavior was absolutely reprehensible. Alas, ~20 years ago, I did (more or less) the same thing myself, though mostly innocently.

I was flying lug(Lugano)-zrh-bos on SR, and had no intention of sitting anywhere but my assigned seat in the back. The FA at the door of the zrh-bos flight asked for my boarding pass, and I unknowingly (really!)fished out the lug-zrh bp, which had the seat number (8B) in really big type, and the flight number and city pair in really small type. The FA smiled and pointed me to 8B, which I occupied for the flight.

Now, I admit that I immeditely realized my mistake. And I am sure that all, or most, or several, of you would have Done The Right Thing and gone back to 47H or wherever. Moreover, I would strongly discourage any of you from trying this, and I would be brokenhearted if I thought that even one of you would be corrupted by the idea.