Dear Minnu,
Please understand that this is the cancellation policy of almost all the carriers operating in this region. The simple logic that their GDS (Global Distribution System) reads the transaction as a single one. So whether you book a one way ticket or a return...if you cancel the route travel, you will definitely need to pay the difference amount as applicable for the whole ticket journey.

Please don't compare IATA rules with IRCTC rules. Lalu Prasad is history...! :)