When my son was in the 7th grade he was given a creative writing assignment. He has always been imaginitive and he writes stories, some for himself , some for others. He got his paper back with a "B" and a hand written message, "This seems beyond the writing of 13 year old. Plagiarism is a serious offense." I was livid. I set up a meeting with the teacher and vice principal. I asked her what proof she had and she admitted she didn't have any. I showed her and the VP other samples of my son's writings. They are very good. She turned red in the face, apologized and reached for the paper on which she had written her comment. I took it and put it in my file and in my briefcase and told her, "Accusing someone of plagairism is a very serious offense. I'll keep this".
I agree, stealing someones' work is worse than stealing money, property, etc. You are stealing someone's creation, imagination, blood, sweat and tears (sorry no citation).