There are no "sides" to this. The seat reclines. The customer pays for that and also knows someone reclining into them can happen as well.
Is it right? Fuck no. But it is what it is and the blame rests solely on the airline for allowing it and/or not creating more leg room.
That device should be banned and people defending the guy using the device are being illogical. No one has the right to prevent someone from using their seat in a way they paid for. Even if it is ultimately using it in a way that makes you an asshole. Which it often does.
Pretty much.