Why isn't Ukraine allowed to use cluster bombs on its own soil against an invader? Russia is using them and is not a signatory to any convention banning their use. They have no legitimate beef.
A real world example: Ukraine is attacking Russian defensive positions across a heavily mined area, they use cluster bombs over the minefield to explode large anti tank mines and explode clusters over Ru defensive trenches, thus improving their chance of success.
What is the down side? The area probably has fewer mines than it did originally and it had to be carefully cleared anyway and the explosives are smaller.