This part is binding upon, and enforceable against, the individual cities and counties which are member agencies of the regional agency. However, an agreement adopted pursuant to this article may apportion responsibilities for the implementation of this part among the cities and counties which are member agencies of the regional agency. Nothing in this section is intended to prohibit a city or county which is a member agency of a regional agency from preparing and submitting to the board for review and approval a source reduction and recycling element or household hazardous waste element.