[corresation]

Russia are not harboring him because he's not in Russia

So what happens if you commit a major crime in that area? Who responds to a terrorist event there?

I don't know specific international laws, but I would guess that Russia has such an area for her own convenience (that if you are just passing through there's no need to bother with forcing you through Russian customs). But I would guess that Russia has every ability (and every legal right) to do whatever they want in that area whenever they want to, but is choosing not to on its own accord.