Is a typical marshal going to know the difference between a bog standard covert radio ear piece or in ear pro like an Invisio X5 headset? Nope. If a customer shows said radio ear piece or even a set of regular earphones and for whatever reason passes them off as ear pro (we know what airsofters are like) and a marshal gives the OK and there's an issue later that day...Is that negligence on the site's behalf? The marshal? Would insurance get involved? Whatever the outcome, it probably wouldn't be good for the site.
But yeah, ear pro should be thoroughly recommended, especially at indoor sites. I had a Tag round go off 1.2m from my head at the weekend. I was glad to have had ear pro on.