I intend to book a one way ticket from Europe to Los Angeles with Delta (operated by KLM), but since round trip ticket is cheaper, is it legal to book a refundable roundtrip ticket and cancel the second leg of the reservation after flying the first leg?

If so, they will refund half of the fare after the first leg is flown? Or they have some other way to calculate the amount of refund? It would not make sense if the amount of refund of 2nd leg of the reservation is a lot more than half of the fare paid.