When I taught photography at various workshops I always made it a point to dissuade my students from taking pictures of other people’s artwork. I did this because I felt it to be a form of plagiarism.
I exclude graffiti because in most cases the graffiti will disappear, either covered by later layers or washed away by those offended by it.
I have always felt that one of the functions of photography is to bear witness without comment or cleverness to what may be gone someday. That is a legacy we leave to the future—simple documentary evidence. Because of that, it became important to photograph it as a record of its existence.