Okay, I respect that some people are simply concerned with precision of language, and that's fine. But:
1) There is a matter of practicality and convenience here. Sure, it would be more accurate if every thread, article, blog, film festival, convention, and bookstore shelf was labeled "Science Fiction, Fantasy, Horror, Miscellaenous Weird Shit, and Assorted Combinations and Permutations Thereof," but that's a bit of a mouthful. Sometimes it's just an easier to put up a sign saying "Science Fiction Section." In the immortal words of Saki: "An ounce of inaccuracy saves a ton of explanation."
2) A mere concern with precision of language doesn't really explain the endless blustery indignation that tends to erupt online whenever someone (gasp!) lumps Buffy
in with Babylon-5.
Explicit or implied is an attitude that "real" science fiction is somehow intellectually superior to all that wizards and vampire crap. From where I'm sitting, there almost seems to be a kind of seige mentality on the part of some sf purists, as though they're afraid that acknowledging any kinship to fantasy or horror (or comic books) is going to give them cooties.
(Oddly, this sort of sf snobbery only seems to flow one way. Maybe I'm just hanging out at the wrong websites, but I seldom see fantasy fans huffily distancing themselves from all those damn robots and aliens.)
Speaking as someone who grew up on Lovecraft, Fritz Leiber, Poul Anderson, Theodore Sturgeon, Richard Matheson, Edgar Rice Burroughs, John Wyndham and
Isaac Asimov and Arthur C. Clarke, I'm not sure why some many fans seem determined to man the barricades to protect sf's precious bodily fluids from contamination . . . .