What frustrates and upsets me is that before Snowden, I would have looked at this as a fluff piece about technology, with some mild nagging doubts about how it could be misused.
Now I see them as NSA whitewashing propaganda, with mild nagging doubts that maybe the original poster had no agenda and it really is a tech fluff article.
I choose to remain optimistic that it does NOT happen all the time because they do not look at the contents of your email all the time. In other words, someone was diagnosing an algorithm (say, how to choose advertisements using the content of attached images), the images triggered the offensive filter, engineer took a look, and reported it.
Perhaps I am naive, but I simply think that Google does not do this frequently because I don't think they look at email frequently, or scan for naughty pics on purpose. As a sysadmin, I generally don't give a fuck what's in my user's email. I doubt they do either, except to advertise to it.
I bet the human brain is a kludge. -- Marvin Minsky