Kidney regulation of osmolarity and absorption of Na and water

Why is the distal tubule/collecting duct the only place to regulate osmolarity, seeing as we can absorb Na without H20 in the ascending loop? Is it just because the effect of any change in osmolarity earlier in the tubule gets undone (by ADH) in the distal tubule? Or is it because those earlier Na transporters aren't under physiological control?