Let me preface this by saying that I am not 100% sure of my response (but pretty darn close). That being said, since you are charged for all of the items (and their associated shipping costs) at the time of purchase for in stock items and when preorders become available, this means that you have an individual figure with your name on it that is sold just to you stored in their warehouse. Meaning, it cannot be sold out from under you. That's why there is a 6 month limit on POL so they can do something with the space they were holding your figure.
It's kind of like prepaid layaway and they are literally only holding onto it for you to save you some cash on shipping. POL is great, especially if you coordinate your purchases to maximize savings on shipping...assuming you can wait!
Which is sometimes much easier said than done. I have MP Sideswipe waiting in my POL queue and it's driving me crazy because I probably won't have enough preorders to justify shipment until January!