Since the internet protocols and domain naming conventions were created by a US Military agency (DARPA), it should be a somewhat safe bet that they assumed that “.gov”, “.mil”, and “.com” would be US sites.
In taking a look at the orginal specification docs, it looks like this is definitely implied. While listing the now familiar three letter abbreviations for top level domains, they noted the potential that other countries may want to create domains and included the two letter country abbreviations as another type of top level domain independent of “.com” and the like. This would allow them to be administered outside of the control of DARPA which, at the time, was still the authority.
From RFC 920 - Domain Requirements (1984), in the section “Top Level Domain Requirements”:
…
Countries
The English two letter code (alpha-2) identifying a country
according the the ISO Standard for "Codes for the
Representation of Names of Countries" [5].
As yet no country domains have been established. As they are
established information about the administrators and agents
will be made public, and will be listed in subsequent editions
of this memo.
…
I believe that this was right around the time that they split off MILNET from ARPANET and the ARPA group was attempting to shed vestiges of its heritage with the new domain names.
But now looking a later in time, the Network Working Group claims that the intention actually was for “.com” and others to be used by all countries, but in practice they were not.
From RFC 1386 - The US Domain (1992):
…
1.2 Top-Level Domains
The top-level domains in the DNS are EDU, COM, GOV, MIL, ORG, INT,
and NET, and all the 2-letter country codes from the list of
countries in ISO-3166.
Even though the intention was that any educational institution any
where in the world could be registered under the EDU domain, in
practice it has turned out with few exceptions only those in the
United States have registered under EDU, similiary with COM (for
commercial). In other countries, everything is registered under the
2-letter country code, often with some subdivision.
…
Fast forward some more to 1998 when the US government got out of the business of managing domain names (through NSI) and opened up registration to many international registrars. This made it much more convenient for businesses in foreign countries to grab coveted “.com” addresses as registration was decentralized. This did not make it any easier for the “.us” domain, however, as it was still managed by USC and had a very strict and awkward naming convention. This limited the domain use (for the most part) to regional government and educational organizations.
All of this changed two years ago, when the Commerce Department decided sell off administration rights to the “.us” domain in an effort to create a larger namespace for internet addresses (see CNN.com). Now, the limitations of its use have been lifted making it on par with the other top level domains.
So my take is that “.us” would be truly redundant for a US company–something like buying a VHS version of a movie that you already own on DVD. Registration is cheap, however, and you can prevent someone else for taking your business name by grabbing it now.