So..the police are not obligated to protect US citizens..do you think citizens have the right to protect themselves?
Before you argue that it is a one off..there are multiple examples of this and the Supreme Court generally tries to rule consistently.
As per a State Supreme Court Ruling - Warren v District of Colombia - "The court stated that official police personnel and the government employing them owe no duty to victims of criminal acts and thus are not liable for a failure to provide adequate police protection unless a special relationship exists"
And another case, Federal Supreme Court - "The Supreme Court ruled on Monday that the police did not have a constitutional duty to protect a person from harm, even a woman who had obtained a court-issued protective order against a violent husband making an arrest mandatory for a violation." http://www.nytimes.com/2005/06/28/politics/28scotus.html?_r=0
Here's a list of other such rulings - http://www.freerepublic.com/focus/news/1976377/posts