Symmetry is not the only map characteristic that gives advantages to one race over another. Terran is really good at sieging, Zerg is good at expanding early, Protoss is very bad at defending open bases, etc.. The pro-played maps are carefully designed to balance all that, and when they fail the players themselves limit their strategies according to what works and what doesn't on each map.

The players even get to choose which maps they'll play (by eliminating some map if they feel it's unfair) and in which order...