yes the reason is that the node should have no parent before addNode() call.
The problem is that CFlyTreeNode.clone() isn't still public.
We had a small meeting here. The clone method will be public. Product will be tested for a while and get published in several hours.
So finally you should try to use (download the latest build)
if (flytreeview_dragObject.value instanceof CFlyTreeNode)
{
treeview.addNode(flytreeview_dragObject.value.clone());
}