subsidised by full-fare pax.
Staff travel is not subsidised by fare paying passengers. Staff travel is normally subject to load so the staff travel IF there is an empty seat. When staff travel on free tickets, possibly once a year, as a leave perk, they are usually 'firm' rather than sub-load but the priority is low and vulnerable to offloading. The amount paid by staff usually covers all the additional taxes and catering.
I am sick and tired of non aviation professionals who can't hide their jealousy of aviation staff but would guard the perks of their own profession with their life.