If there's cabin decompression and people need to put on oxygen masks quickly to avoid passing out, that tent is going to be a significant hindrance. While it p

If there's cabin decompression and people need to put on oxygen masks quickly to avoid passing out, that tent is going to be a significant hindrance. While it probably won't kill anyone other than him, that's still a major liability for the airline. Just like not wearing your seatbelt is unlikely to harm anyone else but the airlines can still require it, the airlines can force people to limit things which would interfere with their own safety while in the airline's care.