In order to workaround this bug, you can handle the OnClientNodeClicking client-side event of the embeddedTree and cancel the propagation of the event if the node that is about to be selected is disabled:

 protected void Page_Load(object sender, EventArgs e)
 {
        Reporting.EmbeddedTree.OnClientNodeClicking = "OnClientNodeClicking";
 }

<script type="text/javascript">
            function OnClientNodeClicking(sender, args) {
                var nodeToSelect = args.get_node();

                if (!nodeToSelect.get_enabled())
                    args.set_cancel(true);
            }
        </script>