Except as provided in this section, every physician, oral surgeon, dentist, or practitioner licensed and registered in this state has the right to contract with any health service corporation for furnishing general or special medical care, dental care, or optometric care, as the case may be. A corporation may not impose any restriction as to the methods of diagnosis or treatment. The private relationship of physician and patient, dentist and patient, or practitioner and patient is to be maintained at all times and the subscriber has the right of free choice in selecting any physician, oral surgeon, dentist, or practitioner.