I've found it funny when I've made arguments about Google's ad scanning being something I didn't like, and people always came back with "but it's 100% automated and completely anonymous - no human ever looks at your mail".
I think that argument just got settled with this story - and I won.
Now, having said all that as a justification for why I don't use gmail for my personal mail...
Given that Google already regularly scans your email, I'm not going to complain about their actions in this case. For once something useful may have come from it. But given some of the explicit spam I've seen over the years, I'm not sure that I won't change my mind on this at some point over the next few days. There might be potential for some life-ruining mistakes here on the part of either Google or the police. I really need to know more about whether this email triggered a thorough and careful investigation that led to the arrest of the person, or if the email WAS the trigger for his arrest.