Where on earth did Philosophers get the idea that "just in case" means "if and only if" instead of "in the event of"? I ask just in case there's a legitimate reason for the apparently willful muddying of language!
 for example http://www.askphilosophers.org/question/2290
Professor Bloggs is an easy grader, and students flock to his courses in droves, because he will give an A to a student just in case the student turns in all the assignments. The easiest answer to your question (other than showing in my example that the logical understanding of "just in case" is one legitimate sense of this term) is that the other senses cannot be reduced to the sorts of truth-functional connectives that are required in logic. But plainly, this is one of those expressions that can be used in several different ways--and thanks to Peter Smith for giving another clear case of this (where "just in case really means "because")!