Targeting civilians is a war crime no matter the weapons platform used.
The use of cluster munitions is not illegal, as none of the parties here are signatories to the international treaties prohibiting their use.
They are designed as anti-material weapons, whereas a regular cruise or ballistic missile might blow up a building, these are meant to destroy equipment and kill personnel over a wider area.
Their negative connotations come from the weapons mechanism which is releasing miniature bomblets over the targeted area. Some of which have been known to fail to detonate immediately, and subsequently kill or maime civilians who come across them later.