Not sure if it is legal, but if you disobeyed the GA and weren't arrested or convicted, I imagine you'd earn a spot on UA's No Fly List, and for good reason. When there is a diversion, if there is a possibility to continue the flight, the airline wants to take off as soon as possible. Pax exiting and re-entering the plane clash with that goal. And a pax who leaves the plane has to be checked against the those who checked bags. A pax with checked baggage who leaves a plane must have his bags offloaded which adds further delay.