Hi Vikas,
The explanation for this is that the JSP renders at server side, and the switch between drop down values is on client side and so there is no linkage between the two.
To achieve what you request you should use client side eventing in order to populate the TLN.
here you can find the javascript API to get the nodes:
http://help.sap.com/saphelp_nw73/helpdata/en/0a/f84588ea6a4f5c812fdf14f8a03af7/content.htm
you will need to provide the get sub tree action inside of the drop down on change method.
Hope this helped,
BR,
Saar