Yeah I was thinking while reading this- aren’t they actually allowed to sell that empty seat already since they are allowed to oversell? What do they want to do here? Triple dip?
Ideally, prices should be consistent with effective costs.
Practically, I would be content if prices of any single leg couldn't be negative. If leg A costs a and leg B costs b, the cost of A+B can be lower than a+b (you can offer bundles, discounts, etc.); just not less than either a or b.
If you play "tricks", the consumer has the right to use such tricks to their benefit.
I think ticket prices should reflect a combination of the cost of goods sold with a reasonable profit margin, supply and demand, and competitive pressure from market forces, with regulations to prevent fraud and anti-competitive behavior.