In the USA, it depends on jurisdiction and the circumstances. For example, CA is a "two-party consent" state, meaning that to record another party requires their knowledge and consent in some circumstances. This case (a camera in the back of an airline seat) is a bit on the line: the space isn't terribly private (it's a row of seats on an airline), yet a private conversation might be had in a low voice between two people, who did not see, or do not recognize, and were not told that there is a camera pointed right at them.

I do not know if this is a violation of two-party consent. It might not be.