A convenience store clerk refused to sell me tobacco when I declined her request to scan my ID. I did allow her to inspect my picture ID which clearly showed my age to be 63 (as if she couldn't tell by my degenerative condition). She stated it was now state law that everyone gets scanned in California. I can find no such law. No store has requested my ID since. My question is: Do I have the right to demand proof of a law which I believe to be non-existent or interpreted wrong?
The law criminally penalizes anyone who provides tobacco and paraphernalia to anyone under 21, but, "proof that a defendant, or his or her employee or agent, demanded, was shown, and reasonably relied upon evidence of majority shall be defense to any action brought pursuant to this subdivision". There is no clause stating that it is also a defense if a person reasonably believes – without ID having been presented – that a customer is at least 21. So it is discretionarily allowed for a person to refuse to sell tobacco to a person who does not provide ID (there is no law requiring a clerk to take a risk of criminal prosecution).
You can "demand" proof that there is some law (you could "demand" proof that there is a law against selling beer to children if you want). Such a demand is not legally enforceable, that is, if the clerk refuses your demand, there are no legal consequences. A more general statement of the slogan "ignorance of the law is no excuse" is that everyone is expected to know the law, therefore there is in the eyes of the law no need to prove to someone else that there is such a law. There is no law requiring a person to sell you tobacco, and any sale of tobacco to you depends on you finding a person who can legally sell you tobacco and is willing to do so, for some price.
You can argue with them if you can show them the law; or you can speak to the manager (if there is one), to convince them that there is no law explicitly requiring presentation of ID, and the associated negligible (?) risk of prosecution if you happen to actually be a minor.