Cops have no legal duty to protect or serve the public, rather, case law has determined that a cop's only duty is to enforce the law, which wouldn't be a problem if cops actually knew the law, but case law has also determined cops can have a "reasonable" belief that a law exists and enforce it without repurcusion (qualified immunity, decided by Pierson vs. Ray). Cops often escalate situations when there is no reasonable suspicion in order to create a made up crime, violating the 4th amendment through illegal detainment of the person, and get away with it every day. There are 3 levels of cop interaction: voluntary, detainment (involuntary), and arrest (see terry vs ohio). Cops will stop you voluntarily, you have no obligation to comply or stop, but because you don't stop voluntarily, they then have the legal reasonable suspicion needed to detain you for a suspected crime they don't even need a specific crime, just suspecting you of any crime is good enough, and they don't even have to tell you what they suspect you of.

This is all legal, and all affirmed by case law. We live in a police state.