You have to be a US citizen or legal US entity to register a .US domain name. Here is a section of the T&C that covers this:
Registrants in the usTLD must be either:
1. A natural person (i) who is a United States citizen, (ii) a permanent resident of the United States of America or any of its possessions or territories, or (iii) whose primary place of domicile is in the United States of America or any of its possessions [Nexus Category 1],
2. An entity or organization that is (i) incorporated within one of the fifty (50) U.S. states, the District of Columbia, or any of the United States possessions or territories or organized, or (ii) otherwise constituted under the laws of a state of the United States of America, the District of Columbia or any of its possessions or territories (including a federal, state, or local government of the United States, or a political subdivision thereof, and unincorporated organizations based in the United States) [Nexus Category 2], or
3. An entity or organization that has a bona fide presence in the United States of America or any of its possessions or territories. [Nexus Category 3]
Prospective registrant will certify that they have a "bona fide presence in the United States" on the basis of real and substantial lawful contacts with, or lawful activities in, the United States of America. This requirement is intended to ensure that only those individuals or organizations that have a substantive lawful connection to the United States are permitted to register for usTLD domain names.
It shall be a continuing requirement that all usTLD domain name registrants maintain the US Nexus Requirement.