Probably because, like it or not, a certain minimum standard of seating for duty travel is usually a contractual requirement. Given the choice of the jump seat or nothing, the crew member concerned would most likely have been entitled to say 'nothing'.
Fair enough. Have worked for three majors and given the circumstances of the flight in question, refusing the jump seat would have been classified as, "Neglecting the best interests of the company", a possible sacking offence in all three cases.