This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Because in that example the request is being made from an HTML page, outside of the Struts 2 application. Without the extension (by default, anyway) the container wouldn't know it was a Struts 2 action request.