That does seem kind of petty. If you had mentioned it and they refused to move it, then I’d understand the action.
On the other hand, the FA’s actions seem absurd. How hard is it to take the bag, walk forward 2 feet, and say “sir, is this your bag?” instead of putting it outside the plane where the guy might miss the flight he paid for.
The passenger was petty, a petty tyrant and bully. So taking someone else’s leg room and feet space is acceptable so long as no one complains? No I raised the stakes on such behavior. That person will think again before they try to take what isn’t theirs.