You are right! The fix is in qa-page-admin-default.php - change:
if (qa_opt('allow_private_messages'))
array_push($showoptions, 'max_rate_ip_messages', 'max_rate_user_messages');
... to ...
if (qa_opt('allow_private_messages') || qa_opt('allow_user_walls'))
array_push($showoptions, 'max_rate_ip_messages', 'max_rate_user_messages');
Will be rolled into Q2A 1.6.3.