blackjacks and the like are loaded with shot or some other material to provide greater force upon impact.
standard kubotans are just pieces of plastic or resin, and are not impact weapons as defined. they can be used to strike with the ends, but not swung like other impact weapons.
at the point a kubotan becomes large enough to be an impact weapon, it is a baton.
if kubotans are in fact illegal, i am not aware of any law prohibiting them nor am i aware of any interpretation of existing law prohibiting them.