Allowing ajax link clicks to propagate
It's in Drupal.ajax.prototype.eventResponse, round about line 265:

// For radio/checkbox, allow the default event. On IE, this means letting
// it actually check the box.
if (typeof element.type != 'undefined' &amp;&amp; (element.type == 'checkbox' || element.type == 'radio')) {
  return true;
}
else {
  return false;
}


It's hard-coded, but this is JS - you can just monkey-patch Drupal.ajax.prototype.eventResponse and get it to do whatever you like :)