It's quite simple, really.
Unless Burger King knew about it and did not stop it, then how are they resposnsible.
It's the Master/Servant rule from Common Law in England, now the theory of Respondeat Superior where, if your employee does something wrong, the employer is responsible. This is a generalization, of course. This case has more, which has led to claims of negligent hiring, lack of supervision, duty (by Burger King) to the consumer to provide an untainted product..