I was just in California on vacation and heard about this law while I was visiting Stone. Not only can you only fill up a growler with beer from the brewery on the label, you can only fill up the growler with the style listed on the growler. So a growler that says Stone IPA can't be filled up with Stone Arrogant Bastard, etc.
Again, that's a *federal* regulation. In this case, 27 CFR 7.21 and 27 CFR 7.27.
I think you are either misreading the federal law or talking about a different way of using growlers. According to the federal law you linked to, if I walk in with a growler and ask it to be filled it is considered a large glass and not subject to labeling laws.
What are the requirements for growlers that are “glasses?”
When the brewer fills a growler at the tap at the brewpub, and not in advance of sale, we consider the growler as a large glass sold at retail. These growlers are not subject to Federal labeling requirements. Some States consider this bottling activity and regulate accordingly. Brewers should check with State authorities.
In WA, I don't know of very many breweries that fill growlers in advance for sale. Everyone will sell you an empty growler and fill it with the beer of your choice, or they will fill any "growler" you bring in, including mason jars or whatever, they don't care. They can do this because there are no laws covering it in WA, and it is not a violation of federal law as explained above.
The CA law prevents them from filling growlers on the spot that have a different label, but it is not a violation of Federal law.