By doing so the inner logic of the RadTreeView will not change the selection to any of the children, because the event will be invoked every time you click on a RadTreeViewItem. This is why you have to implement custom filtering logic to stop the selection only on specific nodes.
Please keep in mind that the focus of the items is not controlled by this event and you have to deal with it manually, because the clicked item is not going to be selected but it is going to be focused.