If you don't complete a segment, the rest of your itenerary is automatically canceled (you have to call them to rebook; pay fees, etc.). If you only throw away the return trip & don't rebook, while it IS against the airline rules, realistically, people miss flights every day on every flight due to changing circumstances (esp business passengers, the airlines' bread and butter) and don't rebook. Airlines do not bill you for missed flights. This is why airlines routinely over-sell flights.