I have to agree that fantasy and sci-fi are being shelved together, but that's about where I draw the line. I don't really care for it, either, as a consumer, I consider the things to be separate and would appreciate them being shelved separately.
True that there are fantasy novels where "anything can happen" (Harry Potter springs to mind, as that world's magical system seems to have no practical limits. It would seem not knowing the proper words and practicing is the limit on even the direst curses, and a child could master them if they wanted to or had the master that would teach them), alternately, there's what I'd now name Hard Fantasy (Harry Dresden springs to mind) where there feels like a set of laws and systems worked out as to what exactly the limits of magic's ability is (call them magical physics, I suppose), and the world system evolves from there. Those wizards seem to eventually run out of gas, unlike in Harry Potter where you can throw death magic all day.
But I'm not being very fair, because all the HP I've read was the first book. Maybe there are limits, but there don't seem to be. And it also seems like any race can be a wizard in HP.
But I digress.
Similarly, there's soft SF. Less oriented to the rules and physics of things and more oriented to the social effects or the situations that arise. I write a fair bit of soft SF. Mostly I write it because I think a lot of what I come up with is borderline inevitable, and rather than waste story time on explaining how androids or robots exist, I move on. It's a question of focus. I'm not writing so people can check my calculations, and it's fairly tedious to do so anyway, and most readers don't seem to care, let alone the editors. (See the second story Daily Science Fiction sent out-- Mike and Shelly's)... the time dilation sure feels too severe for me.
Hard SF is something else. There's a focus on the technology and a lot of work on the writer's part to develop and build and create plausible explanations for the various technology. The sub-genre has it's merits, but even something 'hard' like The Forever War isn't all that hard when you look at it. The distances are real, the time-dilation, but there's not much focus on the thesis defense of the star ships and so on. Partly this is a result of the author's choice of a first-person narrator. Had a physicist been the narrator for The Forever War, it might have been a more technical book. Although, come to think of it, Mandela is a physicist. But given the time scale of the book, Mandela himself can't keep up and eventually the technology starts to sound like magic.