R.I. Gen. Laws § 27-20.5-1: Restriction on participation prohibited.

It shall be unlawful for any nonprofit medical service corporation, hospital service corporation, health maintenance organization, or any other insurer offering and/or insuring health services on a prepaid basis as defined in § 42-62-4(7), or any other person, to provide that providers of medical services defined in § 27-20-1 shall render those services exclusively to subscribers of any one particular nonprofit or for-profit medical service corporation, hospital service corporation, health maintenance organization, insurers, or any other person in order for the provider to be a participant in the nonprofit or for-profit medical service corporation, hospital service corporation, health maintenance organization, or any other person; provided, that this section shall not apply to medical service providers who voluntarily agree to render those services on a full-time basis to a health maintenance organization.