The short answer is that HYPP is very intertwined with halter horse bloodlines and it is thought some of the extreme muscle mass is actually a result of HYPP. For a long time, it couldn't be tested for. And now that it is, they would have to throw out a lot of the horses if they were out-right banned. So they are trying to encourage people to move in the right direction and not breed carriers if possible.It seems ridiculous that somebody would allow a sick horse to show. Anybody know why they let this happen?