To make this statement in this context shows your lack of understanding of this concept. If they can be sued for every case of burglary, wrecks, robbery, ETC then there would be no Govt at all because there is no way to prevent every crime from happening. This is where this legal language come from, but it is often picked up and used out of context by the more immature in our society...
Regardless, they have zero obligation to protect us.