Form returned by drupal_get_form() won't submit
It's not working because the form builder function user_register_form() is expecting a couple of arguments, $form_id &amp; $form_date, that you're not passing (see the drupal_build_form() docs to know more about what they're for). 

If you want to provide a wrapper function around the user reg form, you can do something like this: 

function my_module_customer_registration($form, &amp;$form_state) {
  $form = user_register_form($form, $form_state);
  return $form;
}