Many people don't understand the why's of something, so when they hear 'don't ever press an Unsubscribe button' they never hit unsubscribe buttons, even for services they know they signed up for. People will remember things when it is most inconvenient for you.

Also, they may have signed up to find out more because they wanted to find out more right then. After they learned more, they no longer cared. Since they no longer cared and there was no presented way right then to indicate they don't care, they may look at further marketing emails as spam, especially when there is a lot of them and not a simple follow up 'we miss you' a couple of weeks later.

Just because you know you're not sending out spam, doesn't mean that what you are sending doesn't look like spam to the receiver.