I imagine that Obsidian got LA/Bioware's express permission to use/leave some of that in the game. That may have showed up in some contract with Bioware and/or LA. And they're giving their source credit, obviously.
Let's turn this around--say you created a game. Would you like someone exporting your stuff to make another game without giving you credit or getting your permission? What if you only make Teen-level games, and someone ported your material to an M game? Your reputation could be affected.
LA wants control over their material, and I think that's reasonable.