Arkane Studios just released the Arx Fatalis source code so it would appear that Bethesda Softworks and Zenimax aren't opposed to such moves in general. However Bethesda has said that they wouldn't release content that wasn't their own.
Yes, i was thinking about the statement that "releasing someone else's unfinished product is not something we would do"
But they do actually own it. What i think they mean is that it's not their own work and doesn't represent their current product quality.
The "old" part is an important one aswell, a handful of craving fans isn't enough incentive to release such an outdated and potentially unplayable... thing. In short my best guess is that it's not in a shape that they're comfortable releasing. I understand that and i'm cool with it, it just would be nice if it happened
BTW Arx Fatalis was freaking great! First RPG i ever played, and one of the few i enjoyed.