I wouldn't mind spam as much if I wasn't constantly told that I had signed up for the 'newsletter' - why not just be honest?
And I know for a fact that I haven't signed up as I always use firstname.lastname@example.org for my email. Caused a bit of confusion with the nice lady from Microsoft who phoned me up asking for my details... :)
That whole sign up thing without signing up is a relativly new (like in the past year or so) tactic in SPAM. A "legitimate" company rents a SPAM list and then sends an email out that says if you do not opt-out of this massage we will assume that you want to receive email from us. Of course you don't even open the message because you didn't ask for it and you don't bother with SPAM. Next thing you know, your address has been rented out to a billion people as an opt-in address because you never opted-out. It's really dispicable.