The EasyPrivacy block list contains an entry that will block the piwik.js file. Of course, when you're self-hosting, it's trivial to serve that file with a non-default name.
That's an interesting choice. I mean, it's not like you can hide from the web server that you are making the request. But then again, I'm assuming -- by the sheer necessity of having a JS file -- that they are collecting some additional metrics not available to the server in the request.
