Well, supporting unsubscribes in transactional emails can sometimes make sense, like with notification or confirmation emails where the user might want the option to not receive them. With others, like receipts or signup confirmations it makes less sense. For now, we presume that you're going to do the right thing and implement unsubscribes if it makes sense for the emails you're sending. We've got a feature coming to make that process easier, but it's still going to be a decision that the sender is going to need to make.