The situation isn't common--I don't know the last time I've been asked for ID to purchase alcohol as someone who is very obviously not underage--but every now and then you'll run across a place that IDs everyone. Presumably, they've been busted for selling to underage and management has just taken judgement out of the hands of waitstaff/bartenders.
the law could be that if the waiter didn't verify the ID
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...
This doesn't change your basic premise (the law is nuts), rather it extends it (the law is even more nuts than you expected).