Originally Posted by
cepheid
For marketing purposes, as was already mentioned. "Direct" flights show up near the top of the list (whether or not they have a stop), while connecting flights (different flight numbers explicitly) may show up lower. UA clearly wants the marketing advantage of the "direct" flight in those markets where it lists such flights. That's why they force a plane swap even when it seems silly to us, the passengers.
Then why not just make the IAD-AMS plane the same as the SFO-IAD plane? It will still be a direct flight and show up at the top of the list and the customer will be less confused - "how can I be on a direct flight if I have to change planes?"