I'm not familiar with the system, but I find it extremely unlikely if >600,000 doctors are able to organise to form a watertight gate without any help from regulation. Surely they control licensing or something?
If you aren't familiar with the system why make the claim that doctors have control because regulation says so?
Because of what your describe in your original post: That scenario is completely implausible, except where enforced by regulation. I don't know what the exact mechanism is, I can just tell that it must be there. It's a game of prisoner's dilemma with 600,000 participants.