QF has been forced to adopt this policy by the US DOT in response to repeated complaints filed by another airline (who will be unnamed but starts with U and ends with A). It is to preclude accidental cabotage. Note that the trans-pac flight on QF must precede the LAX-JFK sector (as the pax can change the routing and not fly QF trans-pac later). Technically the gov't rules allow QF to do anything they want as long as 100% guarantee that no cabotage occurs (with a big penalty for failure to comply). Your US tax dollars at work, keeping the world safe for airlines that start with U.