Expedia appears willing to risk the wrath (and sanctions) of AC for allowing Back-to-Backs (or AC knows what is going on and tolerates it).
Although I stopped the practice long ago (in internet time, of course), in my early days as a frequent business traveller, I frequently booked back-to-back tickets via expedia. Two separate reservations, but at exactly the same time. The sets of b-to-b tickets came in the same envelope. Surely, a real person (not a computer) stuffed the tickets into an envelope and knew exactly what was going on.
The ethics of back-to-back tickets can fill an endless debate. My guess is that AC tolerates the practice at expedia even though it knows full well what they are doing.