If some distributed the source code, what would happen? It's not like people could start releasing new StarCraft games to compete with Blizzard. And it seems unlikely that the source code is going to give someone some kind of an advantage it creating other competing games against Blizzard.
Obviously legally it's Blizzard's right to keep the code private, and there are times where it's advantageous to do so, but this doesn't really seem like one of those times.