Its just not implemented yet. I will be implemented as part of impplementing support for shippable products. Right now we really don't support shippable products unless you can bundle a single shipping price for everyone into the product price. So we need to impement shipping calculations and inventory tracking and other things that relate to physical products. Right now we support download products well and there is no real inventory issue with downloads.
So its not really a bug, its just a not yet implemented feature. The WebStore is pretty basic and still in its early stages of development.
Best,
Joe