Hmm. So Macroevolution refers to the theory and microevolution refers to one of the mechanisms that drives the theory. Why not simply refer to them as "the theory of evolution" and "natural selection" respectively?
Because when you ask most people why they believe in evolution, they usually start with a defense of natural selection. In many people's minds, the two are synonomous, so creationists have to differentiate the two so people understand that while there is scientific basis for natural selection, there is little scientific basis for evolution.