They're just doing their job. Unlike AARP, who appeared, even then, to be acting against the best interest of their constituents, the AMA is dedicated to helping DOCTORS make a living.
The very existence of the AMA is a product of their desire to drive out competition. Yes, most of that competition was quackery. Yes, it probably helped improve our medical care at the time, but the purpose of obstructing even access to chiropractic care does not come from the AMA's desire to make people healthier and better. It comes from their desire to drive all business to their door.
Improving competition does not help doctors. Doctors and hospitals can charge more now, because of the bloat and greed of private insurers than they will be able to charge a government run insurance program - probably. If it is in the best interest of their constitents, then it is their moral obligation to oppose this.
I still think it sucks that this is happening, but stop acting surprised.