I doubt AA has a written policy anywhere (and certainly not a customer-available one) that says people can just happily swap seats across cabins for the heck of it. Do people do it and get away with it? Yes. However, just because something is often done without intervention, does not make it Company policy...anywhere.
Having said all that, as a regular AA complainer, I was appalled that no miles were proactively awarded by anyone in Customer Relations for the “rude” behavior.