I may be wrong... but I think the dot net stuff is only used by the game launcher window, and that was introduced back in a patch for M2TW (maybe the last one.. 1.2?), when they added the survey questions and promos, which require a 'Net connection.

Unless they've changed something, the dot net install shouldn't be required to run the game, but you may have to make a Windows shortcut and point it directly at the .exe file to bypass the launcher. And yeah, this is pretty sloppy programming. If a game company needs Internet access, just program it in-house like Blizzard does, instead of relying on services that not every machine out there has installed.