There are plenty of websites that are only accessible from within the US. Most of them being video on demand or video rental companies that let you download content and the law does not permit them to do so outside the country hence the blocking.
Some webmasters or companies do not want business from outside the country so having visitors from abroad just eats away their resources and the find it easier and cheaper to block outside traffic.
This is also true for some non commercial websites. Some websites block certain countries to prevent spam attacks. There are many such examples. IP level blocking is very easy to do and hardly takes 5 mins to set up. IP range lists for every country are freely available on the internet.
What can you do?
Use a proxy like hidemyass and use a US IP address.
Source : I am a server admin myself.