Attendance field is always visible on 'signup/edit/SID' form

In the following page handler

function signup_edit_page($signup) {
  drupal_set_title(t('Edit signup'));
  return drupal_get_form('signup_edit_form', $signup, 'admin');
}

everybody accessing this page gets 'admin' type and they can alter their attendance status which should not be the case. Ordinary subscribed user should not be able to change his attendance. The signup form on the page view page works correctly though, the attendance field for ordinary user is not displayed.