Jalal making law on everyone will create rift and that's what he wants.
So that people even accept the new law and never raise question on the marriages done before.If he would have made the law by himself stating that this law will be conducted from this day onwards then like adham many more could have questioned the king for being partial but now he will be forced to do that.
I think its just a trick by jalal to put HIS OWN WORDS FROM PEOPLE's MOUTH.