Which means it's not a good or obvious way to tell if something is illegal or not.

Cops don't show up for civil offenses and most criminal offenses (particularly white collar offenses). Cops are also notoriously bad at understanding the law. Funnily, an illegal arrest is a good example of where the cops don't show up to arrest someone for breaking the law.

You are using faulty logic to determine what is and isn't legal.