I'm not a copyright expert either, but I would think it goes one of two ways.
One is that the original rights holder of the IP could sue these binders for profiting off of it.
The other is that they can't because the work is sufficiently transformative, in which case it would fall to he fanfic writer. From there, it probably depends on how they released their work. Some websites might claim ownership of anything published there as part of their ToS. Some authors might explicitly release their works under more open licenses to encourage community involvement. If it was just posted somewhere without addressing these questions (which I would guess is pretty common)... Sounds like a mess for the courts to sort out.