That is actually a different rule: the tickets have to be used in order. Say you booked round trip A to B back to A and decide to drive from A to B. Tada! Your

That is actually a different rule: the tickets have to be used in order. Say you booked round trip A to B back to A and decide to drive from A to B. Tada! Your B to A segment is automatically cancelled as well. In a way this is even more absurd than airlines banning hidden city ticketing.