British Airways has always operated in this manner. It is far cheaper for a long-haul flight to "start" your journey in Dublin (Ireland), Amsterdam (Netherlands), Frankfurt (Germany) or some other European city. Often you can save up to 25% - 50% of a business class fare from London. Works in economy as well but the savings are less. Lots of UK travels flight to Dublin/Amsterdam/Frankfurt, get off the plane and get straight back on the same plane for the return flight. BA crew are relatively used to seeing such behaviour. I fly Thai because my final destination is Kuala Lumpur. Fares in Thai First Class between South East Asia and Europe are some of the cheapest amongst Asian airlines (MH,SQ,CX, etc). I would agree that Thai business class fares can be higher than other airlines like Qatar which also require a change of planes when flying to South East Asia beyond Bangkok.
The commercial logic is that most people want to fly direct and are willing to pay a premium for this. The airline has to offer an incentive, i.e. a discount, for non-Thai passengers to fly Thai and put up with the connection at BKK which can be a real zoo at times in transit.
This has been going on forever. I remember when they announce BA's Concorde flight was going to finish, loads of folks booked an ex-Amsterdam journey connecting to the Concorde flight at Heathrow. This halved the cost of the fare of flying Concorde.