The corporation may enter into contracts for the rendering of hospital service to the subscribers only with hospitals approved by the commissioner. All contracts issued by the corporation to the subscribers shall constitute individually and jointly direct obligations of the hospital or hospitals with which the corporation has contracted for hospital service. The rates charged to the subscribers for hospital service and the rates of payment by the corporation to the contracting hospital or hospitals at all times shall be subject to the approval or disapproval of the commissioner; provided, that in the case of experience-rated group insurance, premium rates and classification of risks need not be filed but shall be maintained by the hospital service corporations and made available for review by the commissioner upon the commissioner’s request.