People are under false impressions about alot of things. In what way is this relevant to a police officer's oath?
Their oath is to uphold the Constitution. (How they manage to do this while enforcing gun control laws escapes me, but that's for another thread.) They have no Constitutional duty to be anyone's bodyguard.
But a lot of people depend on them for exactly that. Many police officers comment to the press, like after a citizen intervened in a crime, "while it happened to work in this case, we don't advise that you fight crime or criminals. Too dangerous, and someone could get hurt. We advise you to submit instead, it's safer for you and everyone else, and call us afterwards, that's what we're here for." Tends to make people think the police are responsible for their safety, no? Well, it's not true.