First, a little background about me, all the rules I know are from actual ad&d. I know about rules changing for NwN, but don't know EVERY change. So I could be wrong.
So, where did I get it from?
From my ad&d books. I checked up on some of it and I was partially wrong.
The Pale master gets the Deathless Mastery feat at level 10, he becomes half a mummy so to speak. (according to my ad&d book) So he doesn't become an outsider. Same goes for an Arcane Archer because these Prestige Classes are more like a job then an actual change of the person.
To clarify, an example of a prestige class that does become an outsider. This is an obvious one btw:
Acolyte of the Skinn - The AotS strips the hide of a demon or devil and fuses it with tis own. After 10 levels of AotS, he becomes an outsider since the skin and himself are joined for life.
So it's not with all classes. The DD becomes a half-dragon, so he is no nonger commen, same principle.
But since my theory no longer stands, perhaps they made those requirements so that a character can't use those weapons since they are too uber good.