Basically because ticket prices have no relation to the airline's cost, only to supply and demand. So when a multi-leg journey happens to serve an underserved

Basically because ticket prices have no relation to the airline's cost, only to supply and demand. So when a multi-leg journey happens to serve an underserved route along the way, airlines want to be able to charge that leg according to its own supply and demand, even though it's a component of a route that has higher supply. IMO it should be illegal. A company can't stop you from buying a car for its parts. They shouldn't be able to do this.