The chances of getting caught were much less 15 or 20 years ago. For instance the poster who mentioned movies advertised in newspapers. You literally would need to search thousands of papers to find that. Now it'd be trivial to write a python scraper that could search for the movies you want to claim.
One summer vacation when I was in grad school, I got a job with ASCAP and I would go to bars and restaurants and apparently at the time 1990, you needed, not only a license, but a specific license that covered so many seats and number of people per establishment.
Quite a lot of the mom and pop places were not covered. The usually biggest mistake was to only get a license from BMI and thinking it covered ASCAP as well.
Today I work in a private club and out club had performance licenses from SESAC, ASCAP BMI and GMR. AND the bands that play need to be covered by all four as well before they can play.
Considering it's a private club, I often wonder who the snitch is when we got called out for GMR.