The police should be interested in criminal offences, not civil matters. Copyright is complicated because (in the UK at least) infringement can be both, but the two aspects get conflated. The criminal offences (broadly) are to do with dealing in infringing items for profit, and it's reasonable that the police pursue people committing such offences.
The issue of whether these things *should* be offences is a separate matter. What we don't want is the police deciding which offences they're going to try to enforce. If society doesn't want criminal copyright infringement then that should be for legislators to decide, not law enforcement.