Many websites like Google, Twitter or GitHub, refuse to show content in cross domain iframe, for secure reason.
It is the browser that refuses to show the content. The behavior is in response to an HTTP header. See: http://stackoverflow.com/a/19843216
If X-Frame-Options is removed from the HTTP stream, the modified page can be displayed. Browser scripting or a 'Man in the middle' attack (on the content provider not the user) are possible approaches to working around the limitation.