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?
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.