You know, once in awhile, I stumble onto some independent bookstore that tries to have separate sf and fantasy sections. It's always a mess, full of inconsistently applied standards and arbitrary filing. It just makes it trickier to find the book or author you're looking for.
This isn't a matter of academic, ivory-tower definitions, but simple practicality. There is too much overlap between the genres, authors, and publishers to expect the average bookstore clerk (or consumer) to figure out where any particular LeGuin, or Poul Anderson, or Gene Wolfe, or Orson Scott Card, or Moorcock title is supposed to go.
Sure, at one end you have the books with the spaceships on the cover and at the other end you have the dragons and elves, but in the middle things get messy. Where does Pern go again? How about Edgar Rice Burroughs? Does The Martian Chronicles
go in one section and The October Country
go in another? Do you put Niven's hard-sf in one section and The Magic Goes Away
in another? Do you put the Dorsai books in sf and The Dragon Knight
books in fantasy, even though they're both by Dickson. And what about Dan Simmons or George R. R. Martin or Fritz Leiber or Theodore Sturgeon or any number of other authors who aren't easily pigeon-holed?
That way madness lies. Pretty soon you're splitting the sf section into alternate history, hard-sf, steampunk, space opera, space fantasy, psychedelia . . . because God forbid everything isn't neatly filed away in its own little category. And nobody knows what's supposed to be where.
It's just not practical, not to mention vaguely anal-retentive.