<Or the Russians could be concerned that next time, terrorists could acquire the gas or self-administer antidotes before they attack.

As soon as the Russians said what it was (or supposedly), plenty of chemical or pharmaceutical experts knew what it was about and what the antidote was. Same goes for the chechens or the iraqis. So, knowledge of gases and their antidotes is widespread. After all, if the terrorists are ready to die anyway, why bother with an antidote?
It's unjustifiable.
Now, an ally (Russia) of the coalition against the "Axis of Evil" is using (also against its own population) a WMD that the US is trying to prevent the Iraqis from using, by forcing the world to admit it's justified to attack them.

Raoul