The fear that most authors have has to do not with the fear that someone will steal their own ideas, but that someone will claim that they had a conversation with someone that kicks off an idea that they then execute upon, only to have that person come back with the claim that their ideas were stolen. That's usually a weak claim in a claims court, but it still represents time, money, and reputational harm that has to be spent defending it in court.
When you create a narrative work, you are in effect designing a world, including a history and culture. A writer who does so becomes the authoritative source (the origin of the word author) about that world, and as such, they become the final arbiter about what is canon and what is not. I had a discussion with Jane Yolen about this one time, talking about her views about J.K. Rowlings (Yolen had written Wizard's Hall about a decade before Harry Potter first came out). Yolen was not that upset about Rowling's using the same plot (she quoted for or five contemporary authors who had used the same device), but was angry that so many aspects of her universe seemed to have been directly plagiarised.
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