If they 20mph thing is true then its just crappy ABS compared to everything else. Actually, HD ABS can make you a better threshold braker. Because it takes so long to disengage you really try to brake just short of engagement, stopping distances improve;-)
I think that this means that only the "linking" is disabled below 20 mph, not ABS altogether. Linked brakes during slow-speed maneuvering would really mess you up in a "Ride Like a Pro" class/competitions when you are dragging your rear brake constantly.