I mean, I'm a big fan of privacy but I consider an airline cabin to be a pretty public place.

I'm already used to security cameras in pretty much every public building I walk in...

...can anyone think of something nefarious that could be done with these cameras?

The only possible "threat" I can think of would be to eavesdrop on someone's laptop/conversation for potential business secrets or otherwise, but the cameras are pointed at someone's face (not screen) and no mention is made of a microphone... although planes used to have those phones (obviously with a microphone) in them for many years.

And in any case, a commercial flight is probably not the best place to be discussing top-secret info anyways.