Starcraft to be three games. Not what you think.

[Kamakazie]

So what's to stop people from buying the Multiplayer one and pirating the Single Player expansions? Not saying I'll do it, just that it seems like a fairly obvious problem as multiplayer is the number one way to prevent piracy.

They could always add new content for multiplayer. Something like new units, new maps that are dependent on new environment types, etc.