Because MoveOn.org either is or works directly with CPUSA.org
They use to have the interactive links right on their site until we started posting them here.
This jpg is from one of their 2003 sites.
Look at the United for Peace sign up at the top left of this screen capture.
Look closely at the Blue on Blue text underneath the 'Continue' button. It says MoveOn.org
Now the action for where you enter your name to sign up goes to:
action="http://moveon.org/wmdpledge/" method="get" name="quicksignup
