I rarely purchase RT as I prefer the flexibility of changing outbound or inbound if better or cheaper itineraries open space or alternative when the airline craters an existing reservation with cancellations or unfavorable schedule changes. The main exception are when using a companion fare ticket (BA, AS) which sometimes require RT or offer more favorable RT pricing.

The contract rights, etc. are just irrelevant. Most airlines (AC included) reserve the right to refuse transport for a host of reasons. As consumers we can reserve our right not to fly carriers that are unreliable or unreasonable. And I'm sure as hell not flying AC (or putting family on AC) if they make a habit of IDB customers who have paid tickets because they want to see the credit card. I don't need the hassle and I don't care how cheap the fare is.