OK, as a native Californian and someone who was in the Bar/Restaurant business, I have a long background on these rules. The state, through its "ABC" division (Alcoholic Beverage Control) has always been strict.
The chain of Bars/restaurants I worked for had multiple large bars on near colleges, universities, sport venues and military bases.
We were one of the first to offer a "bounty" to our doormen for finding fake ID's. Pretty simple, all our doormen got trained by us, we provided the "ID Guide" booklet that has every single Drivers License/ID issued by Federal and State authorities, and since they change every few years, but usually valid for longer than that, you had multiple version out at one time. So that caught a lot of folks, the second check was a signature. If the doorman didn't think that the photo and signature matched, they would need to call a manager. We would then come over, and make a decision. If we agreed it wasn't the right ID for the person, we politely informed them that we are keeping the ID and refusing service/entrance to the establishment. And then offered to have the doorman call the police department if they wanted to dispute the fact. Amazing that no one wanted that called made.
The US Navy Shore Patrol trusted us so much that we were the only bar in town to keep military ID's (The Base Commander had a special regulation made that stated that we were allowed to have temporary custody of the ID's, and didn't have to call them right away, instead we just wrote a small report, enclosed the signature given, and sent ID's in bulk once or twice a week to them).
Both the San Diego Police Department and the ABC sent their new employees, usually "decoys" to use first, to basically show them how it is suppose to happen. We were never told beforehand, but many of us knew the supervisors, so we played along.
But the law is pretty clear, not just for Alcohol, but also tobacco and other regulated items such as lottery tickets.
No matter the age of the person, a purchaser/buyer of a product must have a valid ID on their person at the time of the actual sale.
It must be one of the approved ones listed above.
Failure to do so can cause the server/cashier, to be cited by law enforcement and/or loss of their job. (Similar penalties happen to those who give alcohol to minors, a common law enforcement sting is to have minors hang outside a tore and have them ask a person to buy beers for them, and they get cited if they do).
The actual liquor license owner usually is placed on probation the first time caught, which means more frequent checks, and if they get caught again, they can have their liquor license revoked, and since some of them are worth over $100,000, it is understandable why establishments are so strict.