When a TI is working at a DZ, whether it's regularly or as a fill-in, it's the DZO's responsibility to know who is working for him, therefore the actions and negligence of any TI at a DZ IS the "fault" of the DZO who is allowing them to work there. It's the same in any other job/business. It's the DZO's responsibility to ensure the safety of the students and instructors, and uphold the integrity of his business as well as the sport as a whole.