(I'm not on Google+, so I don't speak from experience. That's just how it sounds to me.)
If something happens to you in a social network, you get an email about it. It's a must for any successful social network of any kind. If you don't have this feature, your social product is most likely going to fail. As a developer, you cannot be afraid to send these types of emails.
As long as you give the option to turn off email notifications from your friends or opt out of the service altogether by deleting the account, then all is well.
It's not about being viral, it's about user re-engagement. Facebook knows this, and that's why you get an email notification when any little action happens to you. This is what allows them to have 200m+ users logging in EVERY SINGLE DAY. I find it completely acceptable.
If you want to launch a social product without this feature, good luck not tanking.
It would be you spamming those people, not google or its service. You added them to the "distribution list" as it were.
Google+ is Facebook on steroids.
In another paragraph, the author lauds how Google+ doesn't clutter your mail inbox: Don't mess with the Gmail inbox. When Buzz invaded my Gmail inbox with a flood of social-networky conversations, my first instinct was "Turn it off!"
I guess the message is that if you sign up, Google+ won't annoy you, but it will give you the tools to annoy your friends so their product can be "viral."