You're confused because you believe that the courts would find 'non public performance' only in the most narrow interpretation 'performance only to a small group of friends or family'.

Millions of people play unlicensed music beyond the narrow interpretation of this statute every day, and there's 200 years of case law that I referred you to that doesn't come anywhere even close to indicating that any court is going to take a narrow view of that statute.

The case law highlights where that 'boundary' is and it's beyond a cop or anyone else playing music from their person, on the job or otherwise.