The .US extension has some residency or commercial presence limitations. You should be either a resident in the USA or have a "bonafide" (legitimate) business operation here.
Here is the official explanation:
"All applicants in the .US expanded namespace are expected to meet (and remain in compliance with) the U.S. Nexus Requirement . For additional policy information, please refer to our main policies page."
Your question seems to pop up in many .US discussion threads on this and other domainer forums. You'll see lots of foreign owned .US domains, many are legit and some others are not.
Some domainers have set up US corporations for domaining purposes. There seems to be a grey area as to what qualifies as a business presence. Look through current NP discussion threads for more of this discussion.
There appears to be sporadic enforcement by the registry who can take away a domain from a registrant who does not meet their official requirements. The most famous example of this was made when the domain "video.us" was taken away from a non-nexus-approved registrant after he paid $75k for it.
There are many great deals on keyword .US names (as evidenced in this sales thread) but if you don't fit the nexus requirements, the prudent thing to do is not to take a chance. You could lose the name.
.