The AMA's opinion is about forensic issues, not just disability. This is based upon their review of the law and professional guidelines. For example, the american academy of psychiatry and law's ethics guidelines state that,
" Treating psychiatrists should therefore generally avoid acting as an expert witness for their patients or performing evaluations of their patients for legal purposes. "
Some references:
American Academy of Psychiatry & Law (2005) Ethics Guidelines for the Practice of Forensic Psychiatry
Hales, R. E., Yudofsky, S. C., & Gabbard GO. (editors) (2008). The American Psychiatric Publishing Textbook of Clinical Psychiatry, Fifth Edition. American Psychiatric Publishing
Melhorn, Talmage, Ackerman, and Hyman. AMA Guides to the Evaluation of Disease and Injury Causation, Second Edition. 2013. American Medical Association.
Reid WH. Treating clinicians and expert testimony. Journal of Practical Psychiatry and Behavioral Health, March, 1998, 4:121-123.