The basic answer is that the US opposes the treaty because despite the “legitimate humanitarian concerns” about the impact of cluster bombs on civilians, “The United States relies on them as an important part of our own defense strategy”. This is a pretty tautological rationale for opposing the treaty. The case for banning cluster munitions is that the use of such munitions as an important part of a country’s defense strategy comes at a high humanitarian price. The US military acknowledges the humanitarian concerns, but wants to use them anyway. Kids blown up by unexploded cluster bombs are just out of luck.
President-Elect Obama has in the past voted to restrict the use of cluster munitions, though we’ll have to see how long those humanitarian considers last when they come into contact with his desire to build a positive relationship with the military brass.