If I click an unsubscribe link and the company wants me to sign in, I mark it as spam in gmail.
It's amazing how many US based startups does this.
If I didn't explicitly opt in (checking an unchecked checkbox, emailing someone and saying "please subscribe me to your ads", etc.), I mark it as spam.
One of the ACM's email lists makes you log in to unsubscribe, but then I can't access that section of the site because my membership lapsed.
And this organization purports to represent the state of the art in this field.