United States and Russia never signed the Convention on Cluster Munitions probably for exactly this reason. There are plenty of war crimes United States commited over the years there is no need to accuse them of something they didn't.
That's usually how international law works. It only becomes mandatory when there's a critical mass of countries that agree. Yay, "society of states in anarchy". /s
It's only a war crime under international law if used against civilians. But it would of been an international crime by all use, production, transfer and stockpiling of cluster munitions if they were part of the convention.