I can't believe unsalted MD5 is "by design" (https://github.com/piwik/piwik/issues/8753).
Considering Piwik is used by the GCHQ, I find it hilarious.
I don't find it using poorly implemented hashing in the administrative interface to be at all relevant to what they're doing, or why they shouldn't be using it.
Given these known security flaws, it's not a stretch to assume anyone who can see the GCHQ's Piwik server can have that data too, regardless of whether they are authorized.
See below for a small preview of what an attacker could exfiltrate (dissident IPs redacted for a reason):
https://firstlook.org/wp-uploads/sites/1/2014/02/piwik2.png
While we're talking about poor security practices: the privileged username in the screenshot is apparently still the default ("admin"), so I hope the password isn't still "changeMe" ... http://piwik.org/faq/how-to/faq_191/
Piwik is still using (unsalted) MD5 for passwords in 2015, and probably will still be using unsalted MD5 in 2016.