This is an interesting thread. It's not so simple as "no one was using it" / "the passenger was asleep anyway."

Common courtesy is to ask before the passenger falls asleep.

it vaguely reminds me of a time about 15 years ago... I was in f with an empty seat next to me. The guy in the row across finished his meal and out the empty tray on the seat next me. I was really annoyed. (Look, it's 15 years later and I still think about it). Was his tray in my seat? No. If he asked, would I have put up a fuss? No. But it's just not something you don't do.

In the case on this thread, we aren't talking about half-eaten food that smells. But we are talking about respect of space.