good question Richard
+1 Greg's comments and, FWIW, add the following:
All ISO setting does for astro is complicate matters by making it seem as though you can make the camera detect dimmer targets by setting the ISO higher. As you point out though, you could get the same result by scaling the signal in an image taken at lower ISO. Provided you have the gain (ISO) set to ensure that the signal on extremely bright targets is just below the RAW saturation level, the dynamic range will be as wide as possible and the SNR will also be as high as possible. Higher ISO does not provide any advantage at all, since you do not need to manipulate depth of field, freeze image motion etc - plus you start to lose dynamic range. Its a pity that cameras do not have a "science" setting which sets the gain to optimise the dynamic range and turns off all pre-processing - shouldn't be difficult.
Just a guess, but I suspect that the nightscape folks use high ISO so they can see what is going on - no point in taking a sequence and only finding during later processing that the focus, depth of field or framing is not what you wanted.
In case you haven't already seen it, this is worth a very careful read
http://www.cloudynights.com/item.php?item_id=2786
regards Ray