Ariz. Const. art. 15, § 12

All charges made for service rendered, or to be rendered, by public service corporations within this state shall be just and reasonable, and no discrimination in charges, service, or facilities shall be made between persons or places for rendering a like and contemporaneous service, except that the granting of free or reduced rate transportation may be authorized by law, or by the corporation commission, to the classes of persons described in the act of Congress approved February 11, 1887, entitled An Act to Regulate Commerce, and the amendments thereto, as those to whom free or reduced rate transportation may be granted.