In my opinion, if matzo balls are allowed for Passover then kitniyot should also be allowed for Passover. I feel the reverse is true: Kitniyot is OK and matzo balls not OK! Please comment with your own explanation.
Matzah-balls are made of matzah-meal, which is effectively ground matzah. Because it has already been baked, contact with water cannot cause this matzah to rise, and become chametz. This is the reason why matzah-balls are permitted on Pesach.
Kitniyos are permitted by Torah and Rabbinic law, but Ashkenazi Jewry took on a custom not to eat legumes, beans, and grains on Pesach, for various reasons (because they can be made into flour, or because they grow in similar ways to the grains that become chametz). Please see a previous post on kitniyos, where we explained that according to most opinions, kitniyos that have not come into contact with water are permitted even according to this custom.