Where on earth did Philosophers get the idea that "just in case" means "if and only if"[1] instead of "in the event of"? I ask just in case there's a legitimate reason for the apparently willful muddying of language! [1] for example http://www.askphilosophers.org/question/2290

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I recall someone sending me a short paper complaining about the linguistic tic of using "just in case" to mean "if and only if" when I first started editing Analysis 20 years ago. So, rightly or wrongly, this has been going on for a while!

But note, we can't grammatically substitute "in the event of" for "just in case" in e.g. "I'll buy some tofu just in case some guests are vegan". And the latter doesn't mean the same as "I'll buy some tofu just in the event that some guests are vegan" either. The first, on my lips, means that I'll buy the stuff anyway, so I'm prepared: the second means I won't buy the stuff unless I really have to.

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")!

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