J. Michael Straczynski had to hunt down a Babylon 5 fan who came up with the same plot for an episode as he did and posted it on a forum that JMS frequented.
That he hunted the fan down is not proof that he was legally obligated to do so.
Taking ideas from fans without credit is a good way to alienate a fan base.
Harlan Ellison's name was added to the Terminator credits after he filed a suit alleging that the story was a mix of two Outer Limits episodes he wrote.
An out of court settlement, particularly such a simple one is not proof that a plaintiff could have won on the merits at trial.
Paul Verhoeven had to acquire the film rights to Starship Troopers after someone pointed out that the script had some basic ideas and plot points that were similar to the book.
The major studios are sensitive about copyright and trademark issues. Often, more sensitive than the law actually requires. It's a professional courtesy issue.
How about superheroes like Superman or Batman? Just character ideas, but still protected by copyright.
You are wrong. Characters, particularly visual comic/tv/movie characters are protected by trademark, not copyright.