They obviously did it because Warcraft III and other games can easily be played via third party programs such as Hamachi and Garena without CD keys - through the LAN mode.
However, I think they forgot that Korea made the game what it is today, and the percentage of Asian gamers who only play at LAN centers is MUCH higher than the rest of the world.
Dumb by them. It wouldn't be necessary if they didn't sell the game in three parts... maybe people would actually buy the game if it was a normal price.