A non-approved TLD advertised mainly via spam, the .USA domain is maintained by a registrar completely unaffiliated with the registrars of .com and the like.
While anyone can, of course, run their own nameserver for a domain that isn't recognized (for example, home LANs might use a .local domain), the problem is that it's not actually useful to anyone.
If you ask your computer to go to www.some-name.com, it can ask any .com root server for help. These root servers are well-known, and most DNS servers already know how to contact them. However, if you ask it to look up www.some-name.usa, the root servers will simply shrug. The .USA domain simply isn't a recognized top-level domain.
As a result, anyone who shells out money for a domain in any unofficial TLD is taking a substantial risk. If the domain's registrar manages to convince the Internet community to change all the root servers, then your domain might eventually work for people all over the Net. More likely, however, you'll have paid these guys money for a domain that for most purposes simply doesn't exist.
It should be pointed out that there is already a .us domain, which is planning to accept registrations of the kind the .USA spammers are hawking. The difference is that .us domains already work just fine. Don't be confused: .us is the domain officially recognized by nameservers across the globe.