I fully understand why the people who have had sections from their stories used in this writer's stories are so upset about it. As a fic writer myself I know how much blood sweat and tears go into every single thing I write. But. It's fanfiction. By it's very definition fanfic writers' are using other people's work.
I have actually had a scene from one of my stories used by another writer, my immediate reaction was to be flattered. I view my fanfiction as a writing excercise. If someone thinks something I wrote was goof enough to copy then that's all good!
That said, if someone plagiarised my original fic I would be devasted. While I work hard at my fanfic writing, my original fic is part of me in a way that fanfic never will be.
I can't explain why there's that difference in my head. Anyways, I've lost track of what I was going to say with this post.
Plagiarism is bad.
That should cover it.