l etoile

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.

If you're still not sure there's such a thing as theft of service, next time you get a massage or a haircut or maybe have a hygenist clean your teeth, try walking out without paying and see what comes of it. Of course, keep in mind that a first class seat costs far more than these items - likely moves you into the grand theft category - so whatever happens is likely to not be as severe.