How would you react if you found out someone plagiarized your work and published it?
Truth told? I'd feel pretty damn good. Of all the doubts and fears in my life, one of the worst is that I'll never write/create anything worth publishing. It would be good to put that to rest.
If we're not talking "real" publication, but more along the line of stealing from somewhere on the internet and posting somewhere else on the internet, I still have a harder time caring than most of my peers. Even back when my thing was art, I always thought it would be kind of cool if someone tried to pass off my pictures as their own (they never did).
A while ago I read the first few chapters of a story on The Pit that had an uncanny resemblance to the beginning of my "Let Me In". I'm still fairly certain that the author borrowed some elements from me, probably subconsciously. Godspeed to her, and I hope she keeps writing until she's more comfortable with generating fresh ideas.
If it had been anything more blatant or sinister than that, I might have been pissed, but as leni_ba pointed out, fandom is ruthless with cheaters, and they're more than likely to be found out sooner or later. It's an act that essentially punishes itself.
I guess I'd ask fanfic writers who have had their works ripped off to be patient with the plagiarists. They're probably young, they're probably new to the scene, and they're probably desperate for attention. It might be a boon to have someone tell them that cheating only makes it worse.