It can't be easily undone if it was "clicked" by an automated process rather than a human being.

And when a user receives an email saying they've been unsubscribed because an automated system prefetched the link, they won't be concerned or worried or confused at all. (Especially when this happens _every time_.

And then the automated system clicks the undo link... Problem solved!

Which fires off a new email thanking them for re-subscribing.

Soon your inbox is full.