MOST surgeons do not join the AMA, so don't use it as a litmus test. It's not speciality organization, and it does not help them professionally. A General Surgeon will generally test into and be inducted into the the
Fellow of American Surgeons, (FACS) and if they're a colorectal specialist, then ALSO the
American Society of Colon and Rectal Surgeons - ASCRS. This is why you will sometimes see, "Dr Joe Blow, M.D., FACS, ASCRS after their name.
By and large, the AMA is an advocacy group. They send out their membership to every one with an MD degree, and the only thing you have to do is pay money. The other groups are ones that not only check your credentials, but you must pass rigorous examinations to get into.