We apologize for the caused inconvenience.
For now RadTreeView does not support this scenario when using RadTreeviewItem without some ItemTemplate , however we deeply appreciate your suggestion and this functionality will be added as soon as possible. Your telerik points are updated.
If you want to accomplish the desired functionality you can use ItemTemplate / ItemTemplateSelector /ItemContainerStyle in order to customize the appearance of the items.
Please find attached a sample project.
I hope that this will help you.
the Telerik team