Crimestoppers (A UK charity) are doing this too, and I wrote them to explain the potential for privacy issues. 'You've outsourced crime victims' privacy to an ad company' was my basic message.
I told both the local police and Crimestoppers how easy Piwik was, and how I thought it was a better idea but I don't think I got my point across. There is a gap in the understanding where the site owner doesn't see the raw data (bot Google does) and so they think it's okay.
Anyhow, interesting privacy issues and it may be that I am overlooking something or being overly cautious.
I posted a related question over on Stack Exchange if anyone's interested in providing some feedback there. http://webmasters.stackexchange.com/q/47069
Maybe you sounded like a salesperson? Piwik was unknown to me until I read it in this comment.