Maybe to clear something up for you...
I'm not sure you fully grasp what these stories being published are about. They are not at all taking Twilight, The Hunger Games, or whatever other book complete with world and characters to continue it. They aren't comparable to The James Potter series.
Apparently, the jury is out on if 50 shades is similar to Twilight, although it's not supernatural, isn't a story about Bella and Edward, and doesn't contain blocks of writing from Twilight.
As for others I've read, they are nothing at all like the story that inspired them. I have read a couple where I didn't read the fic version but knew they had come from fic and couldn't for the life of me see them as anything other than original works.
I find it impossible to believe that those books as published could be presented along with the book that provided inspiration and be seen to infringe on copyright. Something which hasn't been tested yet. However, the lack of action would suggest lawyers agree. At least with any currently on the market. I'm sure someone will try at some point to publish something so similar that it will infringe. When that happens, someone will take action on that book but any ruling will only be about infringement in that book.
I personally don't understand authors who take issue with an unrelated story with different characters which doesn't infringe on their copyright. I know I wouldn't, but it's up to each author to decide for themselves.
I would take issue if someone took one of my books and continued it. That is a completely different scenario(as with the one you've offered as an example). It is obviously infringement and in direct competition, possibly affecting sales.
