This discussion brings up an old point that has been decided several times, over various issues, by the Supreme Court of the USA due to appeal from lower courts.
The gist of it is this: Although governments were created to serve the public welfare they do not have an obligation to respond to every incident, and can literally pick and choose. Most of the court decissions about this have dealt with the police refusing/failing to respond to a reported emergency wherein the courts have as much as said the police can stand and watch someone be murdered and are under no official obligation to prevent the crime from happening. What government has is the power to investigate, arrest, and prosecute after the crime has been commited. But the court findings heavily suggests that this lack of obligation to prevent or deter applies not only to the police but any other governmental inforcement agency such as the fire department and, say, immigration.
It is exactly this policy that the Federal Government is currently using in regard to illegal immigration. They have decided not to prevent the illegal entry of foriegn nationals into the US but, instead, concentrate on persuing a policy of identifying and deporting those who prove to be a crimimal element. Janet Napalitano, on the O'Rielly Factor last week, reported that a record number of illegal immigrants, identified as law breakers, have been deported so far this year (if I remember correctly), even over 2009.
So they are using the president of past court decissions to decline to prevent a crime (illegal immigration) but choose to act upon a crime that is commited within the US (ie, robbery, assault, etc).
Another point is that, here in my local area, there are properties that chose to be excluded from an organized fire prevention district. Thge fire departments have the choice of responding and watching it burn while protecting the adjacent properties that lie within the fire district or, if asked by the owner to put the fire out, to charge a large fee for out of district responce. The reason the out of district option is available here in washingington is that the fire districts are a quasi-governmental agency and, as such, have the ability to place a lien upon the property that remains after the fire.