Remove the publishingMode setting from the access form for everyone except the site admin.

remove-publishing-mode.php

<?php
/**
 * Remove the publishingMode setting from the access form for
 * everyone except the site admin
 */
HookRegistry::register('Form::config::before', function ($hookName, $form) {
	if (!defined('FORM_ACCESS') || $form->id !== FORM_ACCESS) {
		return;
	}
	if (/* user is not an admin */) {
		$form->removeField('publishingMode');
	}
});