The police are absoloutly responsible for knowledge of criminal law. And probably traffic law, local statutes. In fact any law that Officer is responsible to investigate and present evidence.
Pertaining to criminal law a Police Officers resonsibility is to.
1) determine if a crime has been commited.
2) bring the person or corperation responsible for that crime before justice.
3) provide evidence that will tend to convict or tend to exculpate that subject in a court of law.