Sure, they knew, but there are lots of factors that would affect whether or not the flight needed the fuel stop. It may have been dispatched with the expectation that it would be fine but enroute a few extra pounds of fuel were needed pushing them below mins for LAX. It wouldn't take much to put a modeled calculation off (all flight plans are computed models since not all parameters are precisely known). One deviation for a thunder cloud and that busts the flight plan. I don't see any way that AC would dispatch knowing full well that it was going to cost them a bundle to make that extra stop. I'm sure the OP would be on here complaining if he had been off-loaded due to weight restrictions and had to take other flights. Why not stop enroute to fuel, I am sure he would ask...