because $400 is a LOT of money for most people to spend on a game console. (let alone 5 -$600).
The truth is they have already reduced manufacturing costs and since they can afford it, they can give bundles away instead of discounts.
Discounts run the risk of devaluing their machine compared to sony (which in their eyes is equal minus the HD movie play) by simply lowering the price now. Since Sony is so high and there are virtually no PS3s on the shelf they don't have to.
And allegedly GoW sold 1 mil from the 8th to the 25th so I'm guessing it moved a few systems too.
I expect more of the deals all through next year before a price drop, which is also coming. When sony hits their $600 saturation point at 5-10 million, they will be forced to add-in or price drop as well. MS spent a great deal of time and effort making sure that they could always be priced equal to PS3 (when they expected $400 form Sony) and now they know they will always be able to be priced lower than PS3.
It's a dream scenario for MS to compete (nobody said win yet), and the Core system is icing on the cake.
Oh and to answer your question, I'd guess that some of these deals are retailers choice but that MS does cooperate as well with discounts.