The Darkover anthologies were not exactly fanfics. They involved OCs set in the Darkover world, with MZB's encouragement to fill in the gaps in the official novels. I have eight volumes, and her own characters never appear in other people's stories, beyond the odd mention or walkover. It's a crying shame those awesome stories stopped coming because someone went to court over a case of synchronicity.
It's also a different situation because MZB was directly aware of the work in question and its contents which led to the issue as it opened up the possibility that she "stole" the idea for the next work (and to be fair MZB doesn't come out of it particular well either; from the evidence... confused thought it is... we're likely either talking exceptionally sharp business practices or a massive over-reaction). While I'm sure there's a possibility that *insert well known author here* browses fanfiction.net or such a site it's far less verifiable then if they take an active part of the scene... and thus if there is any crossover between fanfiction ideas and what the original author intends to write it's less of a legal issue. It was less a case of copyright/trademark/general IP law and more one of plagiarism.
The IP issues generally relate to monetisation. Different jurisdictions have different laws relating to copyright and trademarks (the two key things here) but a general concept is that one has to keep the "purity" of the IP; one cannot allow the IP to be used by everyone and anyone over a sustained period of time and then
bring a case to defend it against a party... in essence by letting those previous infringements stand one has given up one's claims to the IP. Now, for practical reasons it's generally not expected that every
breach of IP is met with a legal threat and (if necessary) a legal action; it would be a huge waste of an IP owner's (who is frequently also the creator) time and money if they had to go after every piece of fan-fiction or the like which included their IP. But one still has to defend against serious or major breaches.
So what constitutes serious or major?
Well, there's no strict definition. But one that has been taken on is that if the third party is making money from it it's more likely to constitute such a breach. So someone writing a piece of fan-fiction and emailing it to a friend? Not likely to weaken an IP. Turning it into a novel and publishing it? Much more likely to. And as with most things the internet has made this both simpler and more complex. Have you posted it on a blog? Do you make money from the blog? Bang. Or even wider... does someone
make money from the blog/website it's hosted on?
Now, in a similar way to a lack of monetisation not meaning IP hasn't
been breached, having some people monetising their fanfiction or the like doesn't mean an IP has been significantly weakened. But it does hurt it. If an IP holder tries to go after someone who is infringing on their copyright... and for these purposes let's say it's a "big" case where someone has released a novel/game/film based on their IP without their permission... and it is revealed that over the years they've allowed dozens/hundreds/thousands of people to make money from that IP without their permission then their case is weakened. And people don't like weak cases...