They're usually not filtering pro-left or pro-right. They're filtering pro-Russian-faction within the left and right. This usually means extremists that used to have no hope of winning in the pre-Internet days.
These factions are more likely to be beholden to their "troll benefactors" afterwards. But even if they turn out to be ungrateful, the extremism itself is worth pursuing. As you said, confusion and division are useful in their own right.
Add other rogue countries to the mix, and you end up with a nasty filter. We should turn it into white noise, or at least drop its SNR.
These factions are more likely to be beholden to their "troll benefactors" afterwards. But even if they turn out to be ungrateful, the extremism itself is worth pursuing. As you said, confusion and division are useful in their own right.
Add other rogue countries to the mix, and you end up with a nasty filter. We should turn it into white noise, or at least drop its SNR.