Because it does have an impact on timeline A. Characters will be able to travel back and forth between AU Draenor and MU Azeroth (in fact a whole bunch of Iron Horde Orcs coming to our world and trying to ax us to death is what starts the expansion), as well as artifacts, resources, etc.
Basically events on AU Draenor won't rewrite our past, but they do influence our present in exactly the same manner events on Pandaria or Northrend influenced our present.