As Demi says, once the item is sold to the retailer/distributor the price is out of the manufacturer's hands. It is on the retailer to the the price. The manufacturer makes a suggested retai price, but that is not legally binding (or else price fixing has occured). So the options that the manufacturer has for price reductions are:
(1) rebate as currently to the consumer
(2) lower price to retailer initially
(3) offer rebates to retailers for cameras sold
Each has its advantages/disadvantages. The reason that they offer rebates and not price changes is because of currency fluctuation. Each corporate year Nikon (or other camera company in Japan) will set a plan rate for currency exchange. Using that plan rate they will estimate the prices for products and set MSRP's. They will set it high enough that currency fluctuations will not affect their bottom line too much one way or another. If the currency fluctuates in their favor then they offer a rebate to reduce the price to the consumer. Without the rebate, they would be at the mercy of the curency market as they wouldn't be able to change the price they charge their retailers mid year (at least the retailers don't like it if you do).
So changing price mid year won't work. How about retailer rebates? Unfortunately, if they did that then there is no guarantee that it would be passed on to the consumer. So Nikon offers Jessops a £50 rebate on each D3100's sold. Do jessops (a) drop the price of a D3100 by £50, (b) drop it by £25 and pocket the rest, (c) raise their advertisement rates but don't discount them, (d) only drop the price by up to £50 if a customer asks for a price match, (e) etc? So consumer rebates are the most direct way to give money back to the consumer from the manufacturer.
Thanks
Rick