Joystiq.com has published an article in which Blizzard have stated the removal of LAN support for the upcoming StarCraft II is largely due to piracy concerns.

"We don't currently plan to support LAN play with StarCraft II, as we are building Battle.net to be the ideal destination for multiplayer gaming with StarCraft II and future Blizzard Entertainment games. While this was a difficult decision for us, we felt that moving away from LAN play and directing players to our upgraded Battle.net service was the best option to ensure a quality multiplayer experience with StarCraft II and safeguard against piracy."

This unpopular move has spawned at least one petition as fans voice their concerns about the lack of a feature they've come to expect from PC RTS titles. Read more at the Joystiq article.