The Google+ Spam mystery

I’ve lurked on Google+ as the network hadn’t grown big enough for me to gain much use out of it. And then I started to receive updates from people I vaguely knew directly to my work email address. Basically spam. So I decided to run a trial update myself on the service to understand how this would have happened.
Google+
Here is a screenshot of the update that I created. I could see from drafting my update what the problem was, I’ve taken a detailed screen grab of the area just above Peter Cashmore’s head
Google+
This box is ticked by default, I guess Google thinks that this is the equivalent of HoTMaiL’s original viral signature: get your private, free email from Hotmail. However it is interesting to note that even early adopters ignore the tickboxes.