But anti- spam software often fetches all resources in mail header fields automatically, without any action by the user, and there is no mechanical way for a sender to tell whether a request was made automatically by anti-spam software or manually requested by a user. To prevent accidental unsubscriptions, senders return landing pages with a confirmation step to finish the unsubscribe request. A live user would recognize and act on this confirmation step, but an automated system would not. That makes the unsubscription process more complex than a single click.
HTTP: method: safe methods: GETs have to be safe, just in case a machine crawls it.