That doesn't make it especially odd. It's exactly as stated, a labeling thing. Buy/sell laws of hard liquor are totally different. Having grown up in California, I'll still getting used to having to go to a liquor store to get spirits. But at least when you do, the label on the bottle reflects the contents of it - and that's what the law is enforcing.
While I agree it's dumb to not be able to fill blank, unmarked growlers, I do understand not being able to fill other brewery's growlers, and if I was a brewer, I would probably support that. I wouldn't want to put my beer into another brewer's growler. And as I'm sure you're realizing, it's not strictly a California thing. Most states' ABC has a similar law.