The auto manufacturers can afford to eat into their normal profit margins and sell the cars at a much lower price (after deals) than the franchisees can, so it would be difficult to craft legislation that protects franchisees without allowing the manufacturers to massively undercut. The only real way to avoid abuse is to prevent auto manufacturers from selling in the first place
I do have to point out that lots of companies in lots of industries (including most B2B technology firms) sell both direct and through partners. Every one of them has to deal with "channel conflict" but that's what you do. It's a balancing act. The partners are never 100% happy but they add value to the manufacturer (or they wouldn't be allowed to stay partners) and the manufacturer can't routinely screw them over (or they'll depart for greener pastures). I'm guessing a mixed model wouldn't really make sense with autos though.
