Originally Posted by Adam Smith

​Remember that they don't price by segment, they price origin to destination. If they charged by segment, i.e. the price for YXC-YVR-YCD = YXC-YVR price + YVR-YCD price, yes, it would be odd. But your YXC-YVR-YCD was priced as YXC-YCD.

I don't know what you paid for your trip, but sometimes onward connections are cheaper than direct flights - in this case, that would mean YXC-YVR-YCD was cheaper than YXC-YVR. It's actually not uncommon for this to happen. Some people choose to take advantage of this - hidden-city ticketing.

If AC were to allow everyone to drop segments whenever they liked, it would effectively make hidden-city ticketing both permissible and quite easy.

Internationally, round trips are also frequently cheaper than one-way tickets, so if you let people drop the returns, it would erode the huge premium they currently make.

Anyway, none of this may be applicable to your particular case since it sounds like your plans have simply changed, but that's part of why those rules exist.