"Der Richter sagt, es ist nicht legal."
Übersetzung:The judge says it is not legal.
Sure, when a normal person says it, I would prefere 'sei'. But the jugde says what is right and what is wrong. He/She decide it, he/she is right. --> One can use 'ist', here. When you are not sure if it is true or when you don't want to give a statement how you think about it, one will use 'sei'.