I think what Gandalf said was wrong..
As for I know, Alot of single player. (Yes, Unreal 1, 2 never had multiplayer before and they wher made outside the developers.)
If the author of the game does not allow MP, then mods wouldnt be allowed. And for what game havnt u seen a mod? or a multiplayer mod?
If it wasnt allowed by the developers, It was not allowed to make SA:MP for example.
So, Gandal is wrong.
SA:MP would be shut down.
Most single palyer games btw who are ONLY developed to be single player its because of the possibilitys offline allready.
I have never heard a multiplayer mod being canceled by the developer of the game.