Here is my idea of how they can do lan play.
Force one user to authenticate with Blizzards Battle.net service.
Blizzard authorizes that user to host a lan game. Once a lan game is
initiated then other fellow lan users can connect. Once connected the
Host game can check versions and whatever else blizzard might use to
ensure it is a legit copy of the game that wants to connect to the
Host game computer. If that host game (connected to battle.net) feels
the client is not in compliance then it will dump the user. If it
feels like it is in compliance then it would allow the user to stay
connected for the game. Once the game is started the host can ether
stay connected or disconnect during the game play and then re connect
once the game is over or the host leaves and dumps all the players.
ether way Blizzard gets one legit copy to connect to battle.net. I just think the development department couldn't come up with a good enough idea or didn't want to.
I wanted to post this on blizzards site, but I lost my cd key for my blizzard games long ago.. let alone find the darn games.