It's for the holidays probably. MS said that they will keep a $50 differrence in MSRP and last Holiday the S was going for $180. They probably want to hit that $130 pricepoint for the late gen buyers.
And not only the holidays. MS keeps having $50 off-1 week promotions pretty every month.
And that without taking into account that we are talking about MSRP here. And since the regular X1S pretty much always can be found for $249 if you look around, I would assume in a couple of months you'll be able to find the SAD edition for around $199 pretty easily.