This happens at the dnn.jquery.js script (/* DNN customized checkbox/radiobox */). It makes changes for all checkboxes and radiobuttons: controls are hidden, but instead it shows styled span.
It looks beauty and great, but sometimes i need simple HTML checkboxes, like this . Had to digg a lot, but have not found where is a problem. So had to check myself and found easy fix. All you need to show simple checkbox or radiobox, just add class="normalCheckBox" or "normalRadioButton" and your controls will be standard.
Enjoy :)