Several years ago I needed to meet Mrs Submareener in Dallas. One way from Tucson to DFW was more expensive than a round trip so I booked a RT and skipped the return.
Look.. I gave you the money so I can use it or not use it...why it should be cheaper to buy a longer flight and not use part of it is silly.. but you sold it.. so I have the right to NOT USE what what you sold me, or NOT USE part of what you sold me..