No, you are misunderstanding the contract. You are not buying a ticket LHR-AMS-KUL. You are buying a ticket LHR-KUL. The fact that the routing or indeed the flight operates through AMS does not give you the right to start or finish your journey there. In order to do that, you would have to contract to do that, which would normally cost you more money. You are then paying for the flexibility. Of course if that was the purpose you would simply buy a ticket AMS-KUL-AMS which would probably cost less or a similar amount with no restrictions.
If you purchase an unrestricted fare ticket, then of course you are entitled to a refund for the unused portions. If on the other hand you take advantage of a reduced roundtrip fare, then that reduction would be as a result of the restrictions you have entered into a contract for. If you have purchased a restricted ticket, then you are bound by the restrictions. Even if that were not the case, the airline would be entitled to deduct the cost of providing the transportation you actually undertook. In many cases a roundtrip or indirect route ticket will cost less or little more than the cost of an unrestricted one way ticket, so you would be at a nett loss.
In the case you cite, you have not bought an LHR-AMS unrestricted ticket, which you elect to cancel. You have bought a LHR-KUL ticket, which you are failing to honour the terms of. The airline in its conditions of carriage makes it quite clear that in doing so you forfeit the value and validity of the ticket and all of the coupons contained within it. Even
if the terms were deemed invalid, the airline would have the right to apply the fare prevailing at the time of travel for the journey it actually provided, i.e AMS-KUL at the prevailing fare.
The fare that the Passenger paid corresponds to the route stated on the Ticket and the usage of the complete routing shown on the Ticket forms an essential part of the Contract of Carriage. The Contract of Carriage excludes the cancellation of individual parts (Coupons) of the journey