"Subscribing in the first place" almost always consists of forgetting to uncheck the "default opt-in" -- also known as opt-out, to normal people. There are also services (including Google) who unilaterally change your preferences from opted-out to opted-in.

Personally, I consider any promotional mail that was not double-opted-in (that is I had to take some affirmative action to receive a confirmation link, and then I confirmed it) to be spam. I also consider any promotional materials stapled to transactional mail to be spam. I know I am not unique in this opinion.