I could imagine situations where airlines need to get pilots/crew/planes to some location for the next flight and somehow recoup costs there, and are willing to cut prices on such multileg flights to take business away from their competitors... but I generally agree with your statement.
That doesn't explain why that would make the price cheaper to keep someone on the flight than letting them get off. It may be a meaningless difference, or even the same price, but cheaper to have the extra leg doesn't make sense if there is competition. That person has weight if nothing else and that costs money to haul.
Yep, it is not just travelers going to destinations that compete for chairs on an airplane but also workers of the airline too. The larger airlines have to balance these priorities
