the ONLY reason I can come up with that Blizz forces everyone to play on B.Net is to "reduce piracy" you have to register your Serial number to your account and thus you can play (even just logging in to solo play) I remember that StarCraft 1, you can play with pirate version on LAN game and only need 1 valid account to update the rest
This whole anti-piracy is not going to fix the anti-piracy IMO. If the game is good enough and worth the money, people WILL buy them. StarCraft 1 are still being sold. People are still buying Team Fortress 2. If people want to pirate, they will pirate and find ways to get around the anti-pirate things. I'm sure someone already figure out how to get around the b.net requirement.