This doesn't change your basic premise (the law is nuts), rather it extends it (the law is even more nuts than you expected).
Not to go all lawyer on you, but wouldn't be a good idea to see what the law actually says instead of making up reasons that might or might not exist? In CA the law is not really that onerous and just says it's a misdemeanor to sell to someone under 21. The only severe penalty is serving alcohol to someone who is a minor and already intoxicated (and thus that much more likely to do something stupid). But it's widely understand socially that any drunk person can be refused further alcohol at the discretion of the seller. But many commercial entities have just decided the way to cover themselves is just demand ID from everyone.
https://www.abc.ca.gov/education/licensee-education/laws-and...
I've lived abroad for sometime now and last time I went home to California to visit, I went to a restaurant to have a beer and dinner with my dad.
I've been gone for so long, I no longer have a drivers license or ID except for my passport, which I don't carry around with me.
I tried to order a beer and they refused. I was very upset as I'm 35 and obviously not a kid. I was just trying to have a beer and catch up with my dad.
Just a huge lack of sensibility in the US these days. They would never do this in 90% of the world. Unless of course you're obviously young.