Actually, if you read the T&Cs on the contract, you really are expected to use the full ride, and the airline is allowed to impose a penalty like this. But it’s at the airline’s discretion, and I think not even the bean counters expect gate agents to enforce it in the most draconian fashion against unaccompanied minors.
This is why they are going to the press with this, because they can’t win the lawsuit. And the Press is the best place to expose behavior that’s legal, but shitty.