In some countries, the law requests websites to provide a way for users to opt-out of all tracking, in particular tracking cookies.
Piwik uses cookies to store some information about visitors between visits. See also: List of cookies used by Piwik.
It is easy to disable tracking cookies by adding one line in the Piwik Javascript code. Edit your Piwik Javascript code as follows:

[...]
// Add this line before the trackPageView() call
_paq.push(['disableCookies']);
_paq.push(['trackPageView']);
[...]