The DAs decide whether to pursue a specific case, but it's the police department that starts the case, not the DA. The DA is not out there patrolling the street

The DAs decide whether to pursue a specific case, but it's the police department that starts the case, not the DA. The DA is not out there patrolling the streets finding people in violation and deciding whether to call the cops. The cops patrol, find a person they don't like, take them to the station, and then the DA can decide whether to charge them with a crime or not.