While there are treaties intended to limit the usage of cluster bombs the Russian Federation is not a signatory on any of those.
Neither is the United States for that matter.
Just a passing thought: the same incentives exist for extremist terrorist groups, if they win they achieve sovereign immunity absolving all past indiscretions, and maybe a cool new emoji in the next iOS update. when people have nothing to lose they have all to win.
it's the common basis of international law. Sovereign nations aren't beholden to anyone else. It's what sovereignty means: they get to do whatever the fuck they want according to their own made-up rules, unless a treaty they have signed explicitly outlaws the thing in question.
There are a lot of treaties governing proper rules of warfare. Russia is entered into many of them. But Russia has not signed any of the treaties outlawing the use of cluster munitions.