Diabetes Insipidus and Na+

Hi,
My nursing books say "should watch for this and that" but they sometimes do not give any rationale as to why I should watch out for some things.

Why is that during Diabetes Insipidus sodium is not being flushed together with H2O. With the amount of water being excreted there should be hyponatremia but it's not. Water follows sodium, and vice versa -- they wouldn't give furosemide (diuretic) to Hypernatremic/ and Hypervolemic patients if it didn't follow. What's the mechanism here? thanks!