So I found this in the comments of FHOTD, it linked to Parelli's facebook:
Our goal at Parelli Natural Horsemanship is to make the world a better place for horses and humans, and a critical component of that goal is safety. We support any protective gear - including helmets - that gives the rider a safer experience, and we require children under the age of 18 and recommend beginners wear helmets no matter the circumstances. We encourage each rider to thoroughly evaluate their situation, enroll in a training program and research and consider all available protective gear and safety procedures.
Glad to see this message makes a bit more sense than their first one..