Why is The Witcher 2: Assassins of Kings free on Xbox 360 in the US? We don't know (CD Projekt Red hasn't updated its Twitter in nearly two weeks)! But you sure can mosey on to the download link and grab it! I just tested it, and can confirm it works at the time of this writing.
I didn't like it nearly as much as The Witcher 3(it's not even close), but it's probably worth playing if you enjoyed Wild Hunt.