I would guess that Kinect, unlike most peripherals, but like most consoles, is sold at or below cost and the profit is made in games sales. If people can use Kinect without buying any games, they reduce the amount of profit Microsoft is able to bring into an already low margin area of their company.
But that also feeds the buzz about the product, which is useful given Microsoft's lack of nerd cred lately.
And some portion of people buying a Kinect for hacks will end up buying an Xbox and games, as well, for when they're not hacking, or for the kids, or whatever.
After all, if they're interested in the Kinect, they'll likely be interested in trying other peoples' applications of the technology - which may include Xbox games.
