Here:
https://github.com/piwik/piwik/blob/master/js/piwik.js#L45

You check for JSON2 object in window object which never exists and you always use JS version of JSON instead of builtin.

Also, here #6066 (comment) man said that you do not use json2.js, but you actually do, or at least json.js which is worse.

Refer to this commend for more details: #6066 (comment)