I think another important consideration is the location of the connecting airport. In your example, no passenger should fly from SYD to NRT connecting via LAX (the advertisement I made for the PEK to SZN flight via NGO in the role-play forum is just for fun and shouldn't have a real market in game).
Perhaps a way to limit it is to calculate for distance traveled: passengers will only connect if the total distance of the individual legs is less than 130% of the distance between the origin and destination airports. Eg. if dist (IAH=DFW) + dist (DFW=LAX) + dist (LAX=NRT) < 1.3 x dist (IAH=NRT) then there will be some connecting passengers; if dist (NRT=LAX) + dist (SYD=LAX) > 1.3 x dist (NRT=SYD) then there is no connecting passengers.
Perhaps a way to limit it is to calculate for distance traveled: passengers will only connect if the total distance of the individual legs is less than 130% of the distance between the origin and destination airports. Eg. if dist (IAH=DFW) + dist (DFW=LAX) + dist (LAX=NRT) < 1.3 x dist (IAH=NRT) then there will be some connecting passengers; if dist (NRT=LAX) + dist (SYD=LAX) > 1.3 x dist (NRT=SYD) then there is no connecting passengers.