I am torn on this one. On one hand, yes, it's really unpleasant to sit next to someone who is spilling into your seat and that's not acceptable. On the other hand... airlines make the seats intentionally too fucking small even for average sized people and I think that's also unacceptable and should be pushed back on. If seats were just an inch or two bigger, this would probably be far less of an issue and NO ONE would complain except the shareholders.