They need you to be online to stop people from stealing maps.
Duh.
A huge part of star2 is the editor. Blizzard wants to see what we can make. They want this map making/buying system in place to get more people to do high quality stuff. If maps were just floating around, it would mess this idea up.
This anti piracy thing is Bull-Shiiiiite, Iccup already has a fake b.net up, and the client is free to d/l from a multitude of sources.
So yes, they are being jerks about this, don't think that blizzard is the company we all love, WoW has changed them into an ugly, greedy, and overall stupid monster.