I seriously doubt anyone's knees would be crushed. Using a device to prevent someone from reclining in front of you is super rude. Either politely let someone in front of you know that your body can't handle it if they recline their seat, or book a seat that doesn't have a chair in front of you. Simple as that. I would be super pissed if someone installed one of those things behind me without telling me when I'm expecting to be able to recline.
Quote:
The argument occurred in the Economy Plus section, which provides United passengers up to 5 inches of extra legroom compared with standard coach seats.
I'm pretty sure the guy was just being an asshole.