There's a few ways to answer this question. In a perfect world, the police have 'qualified immunity' for situations where they have a credible belief that their actions will prevent disaster (murder, mayhem, molestation, etc.). In the real world, that's been expanded to remove any trace of accountability. Combine this with 'self review' policies (i.e. where police review themselves and, surprise, find themselves at no blame).

Also in the real world, the Supreme Court has essentially composed a set of rulings that end up reflecting that the police requirements are to protect property, specifically high-value (i.e. rich) property - they have explicitly stated that the police have no obligation stated or implied to protect/assist/aid citizens.