Horror and fantasy are pretty easy to define so keeping the integrity of science fiction intact shouldn't be too hard though nobody can define what science fiction is as everything is a science and fantastic and horrible as well.
I don't think they are that easy to define, especially horror. The majority of horror stories (novels, books, TV shows/movies) tend contain elements of fantasy or sci-fi. We've gotten horror stories about genetically engineered monsters, witches, demons, and a lot of other stuff like that, that could can also be used in sci-fi and fantasy stories. So most horror stories could also be categorized as one of those two genres too. Typically you tend to see the three genres lumped together in a lot of places, so I don't see why it's a problem that they show "horror" and fantasy on Syfy.