If they did that, it would defeat the whole purpose of the roundtrip booking requirement. Why book a one-way award when you could book a roundtrip (with random return) and then cancel return in order to get a cheaper one-way award? This is also true with revenue fares that have a roundtrip booking requirement. You can't book a roundtrip and cancel return and get full credit for return fare unless you meet the original roundtrip booking requirements for return. If you book roundtrip refundable fares, they also won't refund full value of return fare if you cancel after having taken outbound fare (they will reprice outbound as a one-way fare).