If you do need to add extended fields when creating a user manually, this should do it. Your method, if it worked, would wipe out any existing extended fields that might have been created by an extra.
If you're creating the users in the Manager, I would use OnUserFormSave.
/* only act for new users */
if ($mode != modSystemEvent::MODE_NEW) return;
$profile = $user->getOne('Profile');
if ($profile) {
$extended = $profile->get('extended');
$extended['mailing_list'] = '';
$extended['event_list'] = '';
$extended['template_list'] = '';
$profile->set('extended', $extended);
$profile->save();
}