It makes sense in certain contexts, particularly low crown authority. But if the law of the realm is that vassals can't go to war, then those vassals should not be able to give their own vassals war-making rights that they themselves do not possess.
There's plenty of legitimate reasons to want to be involved with the internal composition of your realm. If you're setting up your children, other family members, or friends with holdings, don't you want to make sure other vassals don't seize them away? And perhaps you want to make sure that one of your ambitious dukes doesn't grow so much that he starts threatening your power. Also, in a more gamey sense, if you're approaching your vassal cap, you probably want to make sure your higher tier vassals stay consolidated so they don't explode into a bunch of smaller vassals and push you over the cap. Etc.