Somehow Grandma's and Twin Cities manage to enforce without complaint. All it takes is positioning a few pairs of volunteers on the course at around the same point (perhaps a mile from the finish) with clipboards and pens for recording bib numbers of people they can clearly see have headphones on and have then hand the lists over to the clerk of the course after the race. Double-check with help from Brightroom or Marathonphoto or whomever the official race photographer may be, since they tend to catalog photos by bib numbers. DQ anyone whose bib number shows up on at least two of the lists. For anyone whose bib numbers show up on the lists for at least two years, bar future entry. This is both simple and easy, it adds essentially nil to the complication of organizing and directing a race.
Yet somehow I suspect that “difficult to enforce” is just a smokescreen and that there were other reasons that would point to compromise and concession.