A service emailing me anything I did not explicitly ask it to is sending me spam - unsolicited commercial email - by your own definition. Just because you find the spam useful doesn't mean it isn't spam.
The key difference is that on SO we don't make you give us your email address to read an answer. So when you "sign up" for the newsletter, you're explicitly signing up for email (in fact, you can do so without even bothering to create an account).
Not saying SO doesn't send too many emails to registered users in other scenarios (it's something we argue about internally fairly often).
And to be fair, both sites put "unsubscribe" links in the footer of every email, so it really shouldn't be that much of a burden.
