The rudder is a distance from the longitudinal centre-line of the aircraft, just as the ailerons are. The ailerons apply a rotational (moment of) force because they are a distance from the centre-line. When you apply rudder, you are similarly applying a rotational force. I suspect that if there were a fin and rudder below and as well as above the centre-line, that the instantaneous secondary roll would occur, but the continued rolling motion would not.