Just because relativity showed Newtonian physics to be wrong about how the universe worked doesn't mean we can't use the latter as a decent approximation under appropriate constraints. Similarly, we don't really need to calculate the probability of the Sun teleporting over Wall Street when valuing Facebook's IPO.

Good question. My best guess is that the law of large numbers averages out the weirdness so that you never spontaneously tunnel through the floor or whatever, but that's only a guess. And there's a lot of weirdness to deal with.