Agreed. "Anything goes" for an initial premise, but the goal is to extrapolate in a well-reasoned way from there. Often the initial premise quickly contradicts itself, like a mathematical proof, and the premise is therefore useless. A good science fiction story takes a workable premise and shows the reader what consequences that premise may have. The premise need not be stricytly scientific either. Larry Niven postulated that the growth of medical transplantation would bring about a black market in body parts, and he came up with the social prediction of "organlegging" - one which in some respects is coming true, although his predictions on its effects on the justice system are, thankfully, not yet apparent. That is the greatest difference between the two branches of speculative fiction. Fantasy is not bound by predictive logic, and focuses on stories in which the bounds of logic are relaxed. Science fiction, on the other hand, centers on logical extrapolations from a premise. Not all spaceship and ray gun stories are science fiction.
I agree. Star Wars is a great example. I like Star Wars, but it is essentially fantasy in space (or maybe fantasy/western) in space. Not science fiction. But in stores that make a shelving distinction between science fiction and fantasy, anything that takes place in space gets put in science fiction whether it is or not. I've even seen Spelljammer novels under science fiction, for those who know what that is. They are clearly fantasy, but hey, space is involved