*Sigh.* We need a copy/paste list at this point.
The strange thing is: this fic appears to be related to the 'Saw' franchise? Which- unless I'm very out of the loop- Mr. Gaiman has nothing to do with. Very strange for such a link to be shared.
Regardless, here, have a bullet point list:
If he looks at/reads any headcanons or fanfics, or even fan theories, he could be accused of taking the ideas and using them for himself. This could result in him being sued for taking the ideas. Avoiding fan works as a whole is the only safe way to circumvent this.
Look. It's simple. Our minds are wild. They can go anywhere, anyplace, anytime. Now, while he has been fully supportive of the creative process that the worlds he's created have inspired, he does not want to know. Believe me. He doesn't. Do you really want to imagine your favourite character that you've created, or one that embodies some secret part of you, in any other way but the way you've imagined them? I would equate it to watching a movie rendition of a book you've really loved. Sometimes it's a beautiful interpretation, sometimes it's so horrifyingly unlike what you imagined that the character is ruined for you forever. Either way, not everyone wants to see that, preferring to leave it to the power of imagination (insert Gene Wilder reference here).
☆ He's asked us not to. Simple as that.
I'm probably missing something, but I hope I covered the basics.
I will say that I come across posts daily that threaten the readers, i.e. 'If you share this can theory/fic, etc. with Neil Gaiman you may wake to find your toes removed' or other equally creative forms of jovial (but serious) threats. It's nice to see the fandoms doing their part, where they can.
Lastly, the amount of kindness and acceptance we get from Mr. Gaiman about the ways we interpret his world is so abundant that I should hope we can take to heart that he is fully accepting of what is talked about and created; as long as he doesn't know about it. It's a simple boundary to impose and the least we can do is respect it.
O.P., I hope this is clear. You may not have known, so I just wanted to explain it. I don't want you to feel attacked by this, so I'm hoping it comes across as mattter-of-factly as possible. We don't need to be hurt any more than our mistakes already hurt us.
P.S. Final note: including this link where he talks about the insanity of the legal world regarding this:
Why I won't read your (or anybody's) fan fiction. Because there are people out there who will decide that I stole their ideas, and who will,