kitniyot

Question

If the kitniyot were mixed before Pesach,while they are still permissible, why can't i do it explicitly for Pesach. It seemed in your reply that it is only mutar if it wasn't done intentionly for Pesach. Why? isn't the heter of why there is no problem of Ein Mebatlin Issur etc. is because before Pesach kitniyot is still Mutar?