It's a sunk cost in every box that ships for the lifetime of the console.
The metro UI works fine with Kinect, but it works just as well with a controller. That's still not a good enough reason to require Kinect with every purchase. They aren't going to release a mainline Forza, Halo, or Fable title that requires motion controls, MS has been the most cautious of the three console manufacturers about implementing motion into their core games, despite the success of Kinect.
They have been hedging their bets with optional modes in a handful of titles, mostly 3rd party, and they're probably gauging consumer response in that regard. Kinect has been a huge moneymaker for them, if it comes down to selling it for $150 vs integrating it at cost ($50?), I think the bean counters win that argument every time. Remember, this is a company that still sells 120GB hdds for over $80, despite it being an integral part of the limited success of the first Xbox and their direct competitor shipping one with every box.