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.
For finding specific authors, anything other than alphabetization isn't practical either, and it's vaguely anal-retentive too. The hack editors, writers and publicists however know quite well that there is a real distinction between various genres and want to have their "genre" stuff separated. Despite the BS above the real impracticality is going through the alphabet by author. You still have to go to the general fiction and YA shelves looking for Ursula LeGuin books, even when you don't bother separating her SF and fantasy!
On the other hand, the hack editors, writers and publicists who lump both SF and fantasy together have already made it difficult to find individual works by unknown authors. It's extremely difficult to find new SF because it's indiscriminately buried in with fantasy (and often horror as well.) It would be interesting that this point has been ignored, repeatedly, except that acceptance of the current system is personally profitable for some. Well, the current system isn't working for some of the rest of us. Trying to dismiss the criticisms with confused drivel displays contempt for the reader.
Also, despite the BS above, it isn't too hard to separate SF and fantasy. There are very few real exceptions. The real cause of freakish misplacements is the usual difficulty that it is hard to categorize unread books. Then the hack editors, writers and publicists confuse booksellers with their self serving marketing.
As to separating books by publisher, who but a hack or someone personally friends with someone at the publisher would even think of such a thing?
The only truly useful categorization of books is between good and bad. Now
that is an impractical goal, wonderful as it would be. This recurrence of the idiotic SF=fantasy comes up because the deliberate annihilation of critical standards is believed or hoped to be profitable. But writing that doesn't even attempt to meet standards, even such supposedly low standards as those of SF, can't achieve much. Hence, the exhaustion. We're really still on the same topic!
PS Peeved as I am at the pissy attitude that it doesn't matter if people like me have trouble finding new books by unknowns, I must admit that the post cited above isn't the sole offender, or even the worst, just the clearest. As such it's just the handiest to take off from.
Also, the SF mode in "technothrillers" blends in with general fiction and gets misfiled too. But there isn't the debilitating schizophrenia between opposites you get with the SF=fantasy nonsense. Hence that somewhat smallish field doesn't get "exhausted."