To put it bluntly: It appears that the US is destroying artillery the US paid for, after it was abandoned to its enemies by troops the US paid to train. Under Saddam Hussein, the Iraqi army had significant heavy weaponry, but much of it was destroyed or decommissioned in the 2003 US invasion. Efforts to provide American-made mobile artillery was seen as a key step toward making the new Iraqi military an independent force. In the past few weeks, millions of dollars of US weapons have fallen into ISIL’s hands.
I guess it didn't occur to anybody that the bad guys with guns could take the guns from the good guys with guns. Or maybe that's a feature.