Well this is Blizzard we're talking about.

And I don't mean Diablo 1 Diablo 2 Starcraft 1 Blizzard, I mean WoW Blizzard. Money, boy. They know if they charge money for people to play Starcraft 2 online, people will still play it, and they will make more money. Monthly fees obviously don't prevent people from playing, just look at WoW.