Not all the Icons face the same way. So it looks a bit odd if you want to have a unified task forced, or two opposing sides, and they're all facing different directions.
A good example are the Icons for the Sath and Collosus. They're both facing left, so if you want a colossus vs. sathannas mission, the two ships cannot face each other in the briefing. Another good example is the Terran capital and Terran Cruiser icons - both face different directions, so if you want to show a Terran taskforce of a destroyer supported by a pair of cruisers moving between nodes, at least one ship type has to move backwards.