You're thinking of state medical boards, but the AMA is a non-profit that engages in extensive lobbying and, via the American Board of Medical Specilities (http://www.abms.org/about-abms/ ), exerts some degree of control of their profession. (I.e., you must join an organization and demonstrate proficiency to call yourself a "board-certified pediatrician.") The AMA calls themselves "more than just a union," and many members are self-employed, but I think it still constitutes a union.

You're right that the AMA has never threatened or carried out violence, but then again, neither have any professional unions I've ever heard of.

For the record, I've spent ~6 years in unionized workplaces and never heard the school teachers, university professors, or government employees who had union cards conspire to commit violence. I would recommend reading about professional unions before decided that they are violent based on "History?".