This is a old "problem", but the explanation is simple.
With airlines you always need to look origin and destination, not the amount of flight.
British Airways may be first choice for most traveler due to the nonstop option and quality brits see in that airline (rather than i.e. Air China). They can sell tickets for a lot of money on that flight, but it will not fill their planes.
So they look at other markets. I.e. Paris-Beijing. There is Air France flying nonstop, and quite a few other carriers, too. Plus a lot of quality carrier offer 1.stop-connections (LH or whatever). So if BA
wants these passengers, they need to have competitive rates on that market, and will offer a way cheaper fare for passengers flying from Paris to Beijing with BA
. Simple as that.
If you book such a ticket and no show on the sector Paris-London your London-Beijing will be automatically cancelled and you will be denied boarding in LHR
. This is a fact and it's written in the fare rules (it's called sequencial use of flight coupons).