Apparently (said a google search some time ago) their insurance only covers them for motor vehicles in the drive-thru, no pedestrians, bicycles, horses etc. The stupidest part is that they dont even warn people not to cycle through, but if you do they tell you its too dangerous to pick up food.
Most of the staff dont seem to know the rules or reasoning and the sign on the windows just says no pedestrians, so sometimes youll get refused and other times youll get a lovely rubbery burger, though i havent worked out in which scenario im winning.