Why let me order an out of stock item?

On Saturday I placed an order online for an item of clothing, and selected delivery for Tuesday morning. I've just received a phone-call telling me the item is out of stock and there's nothing that can be offered as a replacement. I checked online, and the item is still available to order! If there's no stock, please don't waste people's time - rather mark it as unavailable on the website. Now I need to wait for a refund...