It is part of their contract. The airline knows it is part of their contract, and should have an appropriate "don't piss off your customers too much" policy in place for the rare occasion that a pilot takes an F seat from someone already sitting in it. The airline should have trained their ground staff very well in how to deal with this situation. The policy should be in an easy to understand flowchart (i.e., who gets bumped first, what compensation is offered, attempt for voluntary downgrade with comp prior to involuntary, etc - whatever the procedure is) so that even a "day one" hire can follow it.

All that said, and no disrespect to anyone, but (as you all say how can there be no disrespect with a qualifying "but" so soon thereafter) I can't imagine feeling well rested, refreshed and sharp as I can be after a long flight in an AS F seat. I'm just more rested, not as unfresh, and slightly less unsharp than had I been in a Y seat.