If IAH-MIA is short haul according to your definition, then you can justify the use of 735's on that flight. I commute to MIA from SAT each week and the fact that CO insists on using 735's on IAH-MIA route, with the resulting low upgrade percentage, caused me to shift much of my travel to AA. IME IAH-MIA, while not a transcon by any means, is not a short haul, especially if you fly it each week. SAT-IAH in 735 is a short haul, IAH-MIA is not.