In the earlier edition, 15CMS, it was deemed standard usage to denote the United States as U.S. However, that has been changed within 16CMS.
So the U.K. is now UK; the U.S. is now US, when using two-letter designations. 16CMS 10.32.
However, in 16CMS 10.33, there is a distinction made. It is okay to use US for adjectival forms, like in my title to this post. Chicago prefers the full name, United States, for the noun usage.
This is just another example of the dynamic nature of our grammar, and our spelling, rules. So even if you learned it once, it's bound to change later. Or have thirty exceptions. So keep on studying...
Denise Barker, author + blogger + copy editor