When your server responds to the Feed Viewer request*, you agree that the Feed Viewer can display the content, or its part, which your server provides.
When Feed Viewer requests your server for content, it identifies itself* and attaches this License Agreement. By providing content to Feed Viewer by your server, you grant a free license to provided original work to the publisher of Feed Viewer, including the right to display work, or its derivate, publicly.
You can refuse this License Agreement by setting your server to disallow access* to Feed Viewer. Feed Viewer will then not display content of your server.
*) see more technical details at www.feedviewer.net/webmasters