NW in Bangkok is notorious about doing this. Other airlines in BKK seem to do this as well. I think part of this has to do with the fact that once you go through passport control, the airline is obligated to pay your departure tax. I've been given as little as 35 minutes in BKK to go through passport control, run to the gate, and get through security. If you get stuck in the wrong line you could easily miss your flight.