In fact, this orientation used to be considered the correct way. Older zippers are not as stiff and had the capacity to move due to gravity. But if the closed position has the zipper mechanism at the bottom, it can't move any farther down, and just remains closed. ISTR that astronaut and some military flight suits are designed this way, but I could be wrong.

Of course, modern zippers are much stiffer than the ones I'm describing and don't really suffer from that problem.