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.