> What exactly is the worry? Undeserved downvotes?

The Russkies must not get our precious bodily fluids.

I think the steelman for this would be something like: people hear factually incorrect assertions with enough frequency from enough different sources and become convinced that these assertions are true. This is basically of the idea of sockpuppeting/astroturfing. Given that both major political parties in the US deploy such tactics, it is not unreasonable to think that they are effective. And when these tactics are deployed by a hostile foreign power we should be even more upset than when they are used by domestic sources.

OTOH, looking for subtle signs of Kremlin influence in every comment is not without its costs either, one of which is that it tends to make people impervious to evidence or arguments that our policy is too hostile towards the Kremlin.