My guess would be that this came out of MS-Legal. They're probably afraid of a potential anti-trust lawsuit. Granted they would both be accessible as Save As but, I believe theoretically Adobe would still have a case as the XPS is integrated with Vista
out of the box and Acrobat might not be. Also, if XPS appears before PDF in the list (such as the preselected format), they could potentially claim their technology is being restricted from exposure to the end-user. It feels very much like childish wining
to the playground teacher in the school yard but, that's the law.