Interesting pricing policy. Short sighted too. Predatory is the word.
Not using a return leg, even if you cancel, does theoretically expose you to the risk of being charged the full one way fare as the fare rules invariably have a condition such as : "Full and sequential use of flight coupons."
You may get away with it a couple of times but if you do it regularly you will almost certainly find yourself being charged, or the agency you use being sent an an ADM, for which ultimately they will look to you for settlement. I know of cases where this has happened.