It's a law to protect dealers being undercut by the manufacturers selling direct to the public. http://truthonthemarket.com/2013/06/24/tesla-and-the-auto-de...
e.g. why on Earth would anyone hate the existence of physical Apple stores, for example? (which fits the description of "a manufacturer vertically integrating all the way from building [PC's] to running stores.")
Tesla has never had a dealership network, so I don't see why this should apply to them. But that is why these laws came to exist.