The reason is that part of the server's job is to keep an eye and patrons and ensure that they don't "dine and dash"--so in failing to do this, the server is being negligent, or unreasonably careless, vis-a-vis job duties. Furthermore, servers are generally aware that they are resonsible for someone who dines and dashes, so working there with knowledge of the policy can constitute agreement to it.You are also not entirely correct: store employees could be held liable for a theft if it can be shown that their negligence contributed to it (e.g. leaving a door unlocked; leaving the store unattended; leaving money on the counter where someone could take it; etc.).