Any business, person or institution located in the area, which is exempt from the payment of the ordinary business operations tax of the city by reason of the provisions of the laws of the United States or the city or by reason of the provisions of the United States or California Constitutions, is not to be charged under this proceeding, but may make a voluntary contribution to the city. Such contributions shall be used for the purposes provided in this chapter. (Prior code § 23.11.815)