(b) The provisions of subdivision (a) shall not restrict or limit a water corporation furnishing water for fire protection purposes from levying charges for water service or facilities, including water for fire protection purposes, on any person, property, or entity, whether public or private, other than on an entity providing fire protection service. Such charges shall be collected from such other persons, property, or entities pursuant to existing provisions of law which authorize such charges, or from an entity providing fire protection services only pursuant to a written agreement authorizing such charges.