Probably because when they order and pay for what they're getting, their price is set. The yen going up or down after the U.S stores have bought their stock doesn't affect the U.S prices after the fact.
Because it depends on when the retailer locked in to the pre-order/stock price. In other words, you won't see immediate effects on things already on PO in the US, but you can gauge how it's going from any future preorders that will come up.
