GeistSR: In vanilla BG1 there is no dual wielding, but Drizzt is famous for doing so and thus he has both swords on him. You can pickpocket the one he doesn't have equipped. Not really a bug but a consequence of the missing feature.
Hickory: You're right. I just checked his creature file, and Frostbrand is in his amulet slot! It is still a bug (in the engine), because items that are equipped are not supposed to be steal-able. Drizzt has his very own character animation for his dual wielding, so they really should have either put Frostbrand in his inventory, or put 'Defender' in his shield slot (for the AC boost) and Frostbrand in his sword hand. Go figure...
I have always wondered if the developers do that kind of thing on purpose. I guess it's kind of an "easter egg" type of thing? Like, "if you're willing to try it, or just blindly stumble into it, we put some hidden surprises in the game for you" type of thing. Seems many games have these things in them somewhere, and once discovered, they become time-honored exploits.