With the following description:

            <c:simple-property name="shutdownMethod" displayName="Shutdown Method" type="string" required="false" default="JMX" 
                               description="The method used to execute the Shutdown operation;
                                            defaults to 'JMX MBean'.">
                <c:property-options>
                   <c:option value="JMX"    name="JMX MBean" default="true"/>
                   <c:option value="SCRIPT" name="shutdown script"/>
                </c:property-options>                    
            </c:simple-property>                                            

I'd expect checkboxes since required is false, however radio buttons are rendered. This forces the user into a choice even though it's not required (unset is also not checked by default).