How is it the sender's fault if you can't remember why you signed up for an email?
As a matter of best practices, I do remind subscribers. However, it is also quite irritating to be marked as spam, when it was the user who signed up for my messages. By all means unsubscribe, but marking me as spam, especially because I use the highly spam averse Mailchimp is a mean thing to do. Its particularly mean if its just because you forgot what you signed up for.