Curious OP how you think the 'inexperienced' riders are the only ones that need helmets.
So if you mean they should wear one as a preventative, I don't see why experienced riders shouldn't be advised to wear them as well.
Horses are unpredictable and not every inexperienced rider gets themselves into a dangerous situation. Just like every driver isn't going to get into a car crash.
It's like seat belts in a car. Some places have laws, others don't even have them in the car (back in Spain lol) so it's always a choice.
Personally, no one gets in my car without their seatbelt.
As no one rides my horse without a helmet. Beginner or not.
But that's my choice, so if you choose to ride my horse.. you also choose to wear a helmet.