Parece que não estou sozinho ao encontrar esse problema. A questão em questão parece ser que os usuários não internos que são Domain Admins
não são o xelim completo quando se trata de UAC e parecem ser tratados "especialmente":
Windows Server 2008 R2 and the UAC
UAC and Domain Admins permissions issue on Windows 2008 - Part 1
UAC and Domain Admins Permissions Issue or Pocket Full of Kryptonite - Part 2
O parágrafo principal do último link explica:
Basically, [non-built-in users who are - (added by me)] Domain Admins, unlike ALL OTHER USERS, are given two tokens. They have the full access token (like everyone else) and a second access token referred to as the filtered access token. This filtered access token has the administrative powers removed. Explorer.exe (ie the root of all) is started with the filtered access token, and thus everything is started with it.
Think of it is as RUNAS in reverse. Rather than being a Domain Admin you are reduced to peon status. It is, in effect, kryptonite.