Pricing one-ways higher than round trips is a common, decades-old practice. The conditions on the two fares are likely to be different, with the cheap round-trip fare being more restrictive regarding changes/refunds and also probably earning fewer miles. (The one way fare is probably NOT an L fare.)

If you can live with the restrictions, go ahead and save the money. You are not supposed to throw away the return half, but they can't actually force you to fly. No, you cannot get miles for flights you aren't taking.