Oh, that's probably the site itself restricting where it's content can be viewed due to copyright or perhaps export restrictions.
You'd need to use a proxy server based in the U.S. See if
Howto watch ABC TV Streams outside the USA is any help or Google for "US proxy server". Essentially you set the address of one of these servers in Safari's connection settings. Then when you browse, Safari contacts the proxy, the proxy requests the web site page and video, the web site gives it to the proxy, and then the proxy returns it to your browser. I expect these sites also take steps to thwart proxies.
An example of configuring a proxy in Safari is shown halfway down on this
Safari Proxy Setup page. You would enter the server address obtained from the steps above and enter it instead of the "proxy.rice.edu" entry as shown.
I've never used a public proxy and can't recommend one. Perhaps if you repost your question as something like "outside US, what's good US proxy?" someone outside the US with experience will see it.