I don’t think you will find someone on Lemmy who thinks you were in the wrong here. Unless you are buying something that is legally regulated and requires you to identify yourself there is no good reason to be demanding personal identifying information. I always just say “no thank you, I get enough junk mail and spam already” and try to keep it friendly. Every once in awhile they might insist and I tell them I am not going to do that and then they give up. I think most cashiers are at best ambivalent about their employer and don’t really care about enforcing company policy, as long as you are friendly and respectful they would really rather just drop the issue and move on with their job.