^^^ The staff in shops aren't expected to know the intricacies of the law, William. They're simply told that it's illegal for them to make copies of any materials without the consent of the copyright holder. That's why, for example, you can (say) have a birthday cake made with a photograph of your dog on it but you can't have one made with a picture of Mickey Mouse on it.
Any photograph taken in the latter part of the 1940s (from 01/01/1945 onwards) is protected by copyright until 70 years have passed after the death of the photographer. So a photograph from that period, which was taken near to the end of the photographer's life, might no longer be subject to copyright but most photos from that period will still be. A photograph taken before 1945 wouldn't normally still be protected by copyright but, in some cases, it's possible for copyright to be 'revived', so the situation isn't totally clear cut.
Expecting shop assistants to be able to work out the date that a photograph was taken, as well as knowing when the photographer died, is clearly not practical. So they're simply instructed not to copy any photographs unless the person presenting that photograph to them is either the copyright holder or someone who has the consent of the copyright holder to make such a copy.