Doodads dont move im thinking.
The reason you can stack buildings and units, is because of their user inactivity. Blizzard allowed that because in some rare cases (like burrowing too my zerg in the same place) it will crash.
So all interactive units can stack.
doodads dont have this same programming, because they aren't placed by players, and the interactive units have the ability to stack over it. Doodads are like unplacible terrain
