I did find this a bit confusing: "for the CA taxes withheld if there is no tax due?"

Because you stated they have a business (Sched C) in WA, but W2 from California? And there might be no Balance due, or some tax owed to which their prepayment from withholding will be applied. Because withholding is not Taxes; it's just in case of taxes. The States and Fed typically want some prepayment, just in case there will be taxes owed.