It's exactly the same in America, a ticket from New York to Hicksville may involve a transfer in Atlanta onto another flight but costs less than a ticket from New York to Atlanta. Known as "hidden cities ticketing", passengers have been know to buy through tickets and not use the last sector. It only works with hand luggage as checked bags are tagged through to final destination and may be difficult to recover once they are offloaded when you miss the final sector.
The airlines are aware of this and prohibit it in their term and conditions so don't use your frequent flyer account, pay with a rechargable VISA card, change you name slightly eg John Smith/ J Smith/ John C Smith/JC Smith Jnr etc. and have an address and ID in a different state.
If you think BA are bad, try the Middle East airlines, it is sometimes cheaper to fly from Australia to Europe via the ME than it is to fly from Australia to the ME. The reason is that transfer passengers can easily choose another airline with a different hub, but someone going to the Middle East is going there for specific reason and can't change.
Similarly, in the US Miami to Kansas is more expensive than Kansas to Miami, as Miami bound passengers are going on holiday and can switch to another destination if they don't like the price.