Do you know this for a fact? Not sure any military is in the habit of stockpiling weapons they can't use. The weapons have to be maintained, and that's a drain on resources for a resource-constrained military (and the US military absolutely is resource constrained).
Speaking of law, the US cannot legally export cluster munitions unless (a) the weapon has at least a 99% successful detonation rate, and (b) the receiving country can guarantee that they won't be used in areas where there are civilians. The US would have to change its own laws to export cluster munitions to Ukraine.
And all of that ignores international law on the use of cluster weapons. Ukraine use of them would be a propaganda gift to Putin. Also, let's remember that these weapons would be used where Ukrainian civilians still survive and where, hopefully, some degree of normal life may return once the invading orcs are repulsed. Sowing cluster munitions is a recipe for killing a large number of your own population long after the war is over.