My question is really not about open source per se, but the problems it opens for developers just trying to do good works.
The owners of the game deserve to get ripped off because they are stupid.
Even the founders of the modern open source (except for stallman who is a nut) movement state there are times when you should not open source.
Or if you are going to open source, you have to design your business model appropriately.
Basically why you might open source is so you can benefit from the community fixing bugs, improving the software.
Almost all games now days are extended in lua, so the user can make mods etc.
How could the owners ensure that they don't get ripped off ?
In the case of Blizzard with Warcraft,starcraft etc. (they are not open source, but just for illustration purposes)
Even if you pirate the game except for playing the missions the game is useless.
What makes the game interesting is battlenet, and playing other human players.
So even if blizzard open sourced the game, they still have battle net.