Piracy, 'nuff said. LAN kinda encourages piracy, you don't need no original copy to play online. L4d2 is one of the best example

They're out to make money, not charity

There's no such thing as punishing legitimate player for piracy. Consumers are the one pirating their products. What they are doing is simply protecting their properties and justifying the purchase of legit consumers.

How would you, as someone who bought the legit license, think if someone with a pirated copy is playing the same game with the same amount of content without having to pay a single cent out of their pockets? Or perhaps you prefer to have multiple DRM to combat piracy, which it had failed to do time and again but publishers just don't learn?