They have an invested interest in using them carefully and cleaning up the mess ASAP as soon as the war is over.
you’re free to take their word for this but every state fighting a war says that they’re going to use weapons responsibly. how many of them actually do so, or don’t commit war crimes in doing so? the US military, literally the most well funded and powerful combat force in the world (and a force which has not fought for its survival in a long time), still routinely kills civilians in circumstances where that’s avoidable. the idea that Ukraine will be “careful” in its usage of cluster bombs and not misuse them is hopium at best—particularly given the circumstances it’s in. and even if they want to be, again, the point of a cluster bomb is that it’s not a careful munition!
i also don’t think “the bad guys are doing this” is justification for also doing a bad thing that is widely recognized as a crime.