Most things that go under the science fiction tag is often very fantasy-inclined though, which I suppose is why many are going with that speculative fiction tag instead.
Dune is a fine example. There's very little science involved, it's pretty much a fantasy novel but with a couple of planets instead of lands or kingdoms or what-have-yous, so it tends to be labeled as science fiction. Plenty of the classic science fiction writers balance that edge because they never really look into the real science behind what they're writing, so they come up with concepts that sound cool, but which are utter rubbish if you try to look into how they'd really work (of course, in some of the older books' cases the science has changed too, heh)
Of course, science fiction buffs tend to go with the two distinctions of soft and hard science fiction, the latter being those authors who do their hardest not to break the laws of physics with [at this point: fantasy] concepts like faster than light travel or all sorts of cosmic movements that couldn't really occur. But most readers don't care enough about physics etc to actually question what they're reading, so they accept it. I'm one of those readers, incidentally.
But, there's always that knee-jerk snobbiness as well of "but fantasy is just rubbish! Science fiction is like, uhh, almost realistic, kinda!" which tends to really just be based on emotions rather than any real logic.
Not that I've not let my foot fly for the same reason a few times, as I generally find fantasy to be repugnant. I should modify that by say that I've actually started to find some things I enjoy in the fantasy genre . Mervyn Peake, for instance. It just gets easy to equate the genre with all the "brave companions traveling the land" rubbish ala all the Tolkien-acolytes.
A wee tangent: I absolutely hate shopping in these two big stores in Oslo that sell science fiction novels, as I have to wade through ten lurid-looking fantasy books for each science fiction novel. And most of the science fiction novels are rubbish space opera crap or Star Trek/Wars fic etc. Yuck.
Not to mention that most of the books are American imports, and for some reason the SF paperbacks made over there always get the most hideous drawings in the world. Lots of banal drawings of shiny spaceships, rippling muscles and scantily clad buxom ladies.