Steps to reproduce

1. load the zul

   <zk>
   	<tree id="tree" rows="5" multiple="true" checkmark="true">
   		<treecols>
   			<treecol>
                           	<checkbox label="Name"
   				style="font-weight: bold;" id="selectAllCheckbox"
   				onCheck="if(self.checked) tree.selectAll(); else tree.clearSelection();" />
                     </treecol>
   			<treecol label="Description"/>
   		</treecols>
   		<treechildren>
   			<treeitem>
   				<treerow>
   					<treecell label="Item 1"/>
   					<treecell label="Item 1 description"/>
   				</treerow>
   			</treeitem>
   			<treeitem  checkable="false" open="false">
   				<treerow>
   					<treecell label="Item 2"/>
   					<treecell label="Item 2 description"/>
   				</treerow>
   				<treechildren>
   					<treeitem>
   						<treerow>
   							<treecell label="Item 2.1"/>
   						</treerow>
   						<treechildren>
   							<treeitem>
   								<treerow>
   									<treecell label="Item 2.1.1"/>
   								</treerow>
   							</treeitem>
   							<treeitem>
   								<treerow>
   									<treecell label="Item 2.1.2"/>
   								</treerow>
   							</treeitem>
   						</treechildren>
   					</treeitem>
   					<treeitem>
   						<treerow>
   							<treecell label="Item 2.2"/>
   							<treecell label="Item 2.2 is something who cares"/>
   						</treerow>
   					</treeitem>
   				</treechildren>
   			</treeitem>
   		</treechildren>
   	</tree>
   </zk>
   

   http://zkfiddle.org/sample/3m5r5vt/2-Tree-selectAll-not-working

Workaround

                for (Treeitem item: tree.getItems()){
			if (item.isCheckable()){
				tree.toggleItemSelection(item);
			}
		}