It is probably checking the user agent header. Try something like this:
Set the User-Agent request header to masquerade HttpClient as a popular browser such as IE or Firefox. Certain web sites are optimized to work with just one or a number of specific browser applications. These sites frequently reject requests originating from user agents they do not recognize. For example, setting the User-Agent request header to Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) would deceive the target server into believing that the request was issues by Microsoft Internet Explorer 6.0 on Windows 2000.