The US have not signed the 2008 Convention on Cluster Munitions, while Canada and most (all?) other western states have (Russia has not, BTW).
The peculiar stance of the US on this matter should not surprise since there are lots of international treaties that the US have not signed or ratified, including some that one might expect any “decent” nation to uphold [my opinion, of course], such as bans on anti-personnel mines and torture, the Kyoto protocol and many, many others treaties.