I'm going to jump in with only one comment, as someone who not only hold's a bachelor degree in Criminal Justice focused on criminal law and criminal investigation, but also attended the WI Police Accademy:
What @CircleTech said is correct. The police are not required by law, in any way, to put their lives in jeopardy to protect individual citizen of the United States. They are at their core a reactionary, investigative force. They are simply a part of the judicial system, they are not a private security force. Their duties are to serve and protect the public from criminal elements, but not at the cost of deliberately putting themselves in jeopardy of serious bodily harm or death. It is simply not required by law. Their duty is to serve the public by investigating claims of criminal activity and, if reasonable suspicion or probable cause is found to charge someone with a crime, they protect the public by arresting the individual and hold them until they can be processed in the judicial system. The only time that law enforcement officers are directly responsible for any individual's safety is when that individual is in the custody of the State, either because they were arrested and are being held for trial, or because they have specifically requested protection from the State and it has been granted (e.g. the Witness Protection program).
Now, that being said, because law enforcement officers have long carried the tradition of selfless acts that put themselves in harms way to protect the public, it has led the general public to believe that they in fact are required to put themselves in harm's way. This has led a large percentage of the public to expect that they will respond to every call at a moment's notice. They are not required to, and indeed it is impossible for the police in any given jurisdiction, to guarantee that they can respond to every call for help at a moment's notice. To do so would require a police officer to be assigned to every single citizen. Even in cities that are not grossly underfunded and understaffed, there will always be a delay of minutes from the time the call comes in to the dispatch center to when the first officer arrives on scene. This is just one of the many reasons that the US Constitution has the 2nd Amendment. At the end of the day, it is the responsibility of every citizen to guarantee their own personal safety in any moment, it is not the responsibility of the State.