No city or county shall prohibit a person, authorized by one of the agencies in the Department of Consumer Affairs by a license, certificate, or other such means to engage in a particular business, from engaging in that business, occupation, or profession or any portion thereof. Nothing in this section shall prohibit any city or county or city and county from levying a business license tax solely for revenue purposes nor any city or county from levying a license tax solely for the purpose of covering the cost of regulation.