For Gmail's one-click-unsubscribe to function, the subscriber must be able to unsubscribe by sending an email. Linking to a URL will not cause the unsubscribe button to appear.
People clicking Spam! to stop receiving the newsletter is a problem. Clicking the Spam! button hurts our ability to continue to send messages to people who do want to receive it. Unsubscribing will DEFINITELY and IMMEDIATELY remove the user's email address from the available subscribers list.
On Gmail at least, when someone clicks Spam! on a message with a one-click-unsubscribe function, Gmail will ask them if they really intend to Unsubscribe or if they really feel they're being spammed.