RadFormDecorator is designed to work with semantic and valid code, thus it requires a label (i.e. the text property of the server tag) for accessibility purposes. On the other hand, the actual skinning is carried out by the label itself - we change the background image of the label (checked/unchecked) while the "real" checkboxes and radio buttons are sent far left, away from browser's viewport via css.
If your application does not require label, you may add a simple as value of the text property and this will put things to work.
the Telerik team