If you're asking how the bottom 15' is implemented in the wind provisions, its by the Kz factor that is the same from 0-15'
If you're asking why they do that...don't think I can point to anything in the code about it. Plain old simplicity and conservatism probably.
My opinion is that making building engineers buy and study AASHTO in the first place is a huge disservice and waste of everyones time and money. (I'm sure bridge guys feel the same about having to learn a bunch of buildings stuff)
I have no idea what changed between AASHTO versions, but I have...