How do you unsubscribe from certain topics? Would you have to email your friend and ask him or her to not tell you about politics? There's a whole lot of social pressure that means people are reluctant to do that.
So if that person clicked link #1, the token in the link is verified that it was created for that email address, and then it's noted in a database. In the future, if person X sends another email for that category, it won't go to that person. I'm thinking the sender wouldn't even know that the person unsubscribed