Originally Posted by letiole
I don't have time to look up case law for you, perhaps some of the lawyers here could comment, but it's called theft of service and it's just as unlawful and criminal as theft of a tangible item (think about cases that have been successfully prosecuted for theft of cable transission services). Theft of service is when one knowingly secures the performance of a service by deception or threat. If you were caught stealing a F seat and did not move back, you could also be charged with interfering with a flight crew, a federal offense.
There is no
deception or threat involved. Until the flight attendant asks you to move, you've engaged in no deception of any kind. Now, along with other stories people here have posted - if a problem arises and you say something along the lines of "my boarding pass is buried in my carry-on" or if the flight attendant asks you if you are supposed to be traveling in FC today and you reply in the affirmative - THOSE are examples of
deception. If you move to your proper seat when the flight attendant asks you to, without ever uttering any misrepresentation that you are supposed to be traveling in FC, then there is no deception.
It's the same as if right before take-off, I move to an exit row seat that's open. It's not the seat I paid for and I didn't have the capability of reserving that particular seat because I'm not enough of an "elite" - but is it "stealing" ? No.