There's another key difference: it's been legally established that art of a character based on a written description (i.e., a book character) does not infringe copyrights. (Specific example I remember: fanart of Harry Potter based on the book cover or movie depictions might be infringing; fanart of Harry Potter based on J.K. Rowling's written description would not be infringing). Wikimedia Commons has some useful pointers and distinctions in their rules on fan art: https://commons.wikimedia.org/wiki/Commons:Fan_art

Since old-school SFF fandom was heavily based on SFF literature, and a lot of art I've seen at SFF conventions was more generic (dragons, elves, space ships, aliens, starscapes...) and not specific enough to be infringing, that may be why there's a "presumption of innocence" of fanart.