California Business and Professions Code § 25200. Label; Contents.
All beer sold in this State shall have a label affixed to the package or container thereof, upon which shall appear the true and correct name and address of the manufacturer of the beer, and also the true and correct name of the bottler of the beer if other than the manufacturer. No manufacturer, importer, or wholesaler of beer shall use a container or carton as a package or container of a beer other than such beer as is manufactured by the manufacturer whose name or brand of beer appears upon the container or carton, or use as a package or container of a beer a container or carton which bears the name of a manufacturer of beer or the brand of any beer other than those of the manufacturer of the beer contained in the container or carton.
The really irritating part is what I was saying in your other thread where they will only put the same beer in specifically labelled growlers, which doesn't seem to appear in that law.