When your server responds to a Feed Viewer request*, you agree that Feed Viewer can display the response to its users.
When Feed Viewer requests your server for a work, it identifies itself* and attaches this License Agreement. By providing the work to Feed Viewer by your server, you grant a free non-exclusive license to provided original work to all of Feed Viewer users, including the right of Feed Viewer to make a remote copy and display work, or its derivate, publicly.
You can refuse this License Agreement by setting your server to disallow access* to Feed Viewer. Feed Viewer then does not create a remote copy or display the content that your server provides.
*) See more technical details in Technical documentation.