Upvote:4
I would bet there's a line in the small print when you booked the flight about them being allowed to record you at any point for security purposes. Arguably with a growing number of cases of passengers getting drunk and/or abusive with airline staff, a body camera might seem like a good way of making sure there is evidence of exactly what went on in any interaction (even if this doesn't sound like what was going on in your particular flight).
According to Aviation Stack Exchange, many airlines do already have in-cabin monitoring cameras, from which body cams are a small step. Also, this article from the Wall Street Journal mentions an attendant with a body camera on a China Southern flight who appeared to record some western journalists specifically. The official line when asked:
China Southern says the crew member was an air marshal, charged with safety on board.
Generally, I wouldn't think there was an expectation of privacy on a airplane (outside areas like the toilets), and once in the air, the laws of the country where the plane is registered start to apply, which given the Chinese government's approach to surveillance would mean I wouldn't expect any privacy at all.