Sadly, these public musical assaults are usually unpunished by law. Though I did see a very funny New Yorker cartoon once that depicted a burly, unshaven, hulky guy standing on the street with a sandwich board that read: "Kill the street musician of your choice: $5".
As a matter of fact, many, many years ago I spent a summer working as a busker in Italy but we did not perambulate around to outdoor cafes tormenting the patrons. Instead we set up in front of a statue of Cosima de Medici in the middle of a plaza and let people come to us. On a good night there would be over 500 people listening at one time.
Tough to find the right clip, but here is a guy busking outside Notre Dame in Paris. I saw a small chamber orchestra busking in the Metro in Paris at the La Bastille stop once.