I think that email software is not educating people to what is SPAM (un-solicited email) and what is legitimate email, marking emails they don't want to receive anymore as 'SPAM' when they should be saying "Hey, I don't want this information I signed up for anymore! I should click 'unsubscribe'. Of what I've read AOL and Hotmail score these 'mark as Spam' clicks and block senders against that score. This seems like a dangerously inadequate way to determine and Blacklist senders largely down to a failure of the software to educate their users on how they should respond to these types of email or is clicking Spam a lazy and convenient way to not receive this email anymore.
Either way I think it would be great to have an "Un-Subscribe" next to the "SPAM" button and a message that says "It appears that you have subscribed to this email. Would you like to un-subscribe now?".