Sorry if you've never heard of it, Jim, but that is EXACTLY how it works. Airlines are not extracting more money from a customer - they're just using the ticket that was bought for a journey from A to B and back to A for that same journey. It doesn't matter that the flight from B to A isn't on the day originally wanted by the passenger. In all my experience (over 20 years as a manager at a major international airport), we never, ever tried to collect extra for breaking minimum stay requirements etc.,so we merely used whatever ticket was held. However, if they had arrived on a one-way ticket only, we would advise the passenger that the airline was entitled to follow up with a claim for the unpaid flight. We would subsequently complete the internal paperwork, though I seriously doubt that any further action was ever taken by our accounts people!
And as others have said, the responsibility for holding correct documentation always lies 100% with the passenger, not the airline. Any checks that the airline makes on departure are purely to protect themselves against fines or sanctions by the receiving country and not as an additional layer of protection for the passenger, even though refusal to accept a passenger for a flight might have that same outcome.