There is no slippery slope here if the law is states something like: "contracts between existing de-facto oligarchy of car manufacturers and dealers remain as is, new entrants are free to set their own terms".

Everyone is talking about how we can't trust Elon Musk since he is acting in his rational self-interest. Yet why should we assume that Christie is acting out of the pure goodness of the heart? Politicians too act in their self-interest and that kind of self-interest often involves maximization of power. The currency of politics is not just cash (that helps too, of course!), but also favours: you can be sure that in return for this favour, he'll ask something from the dealership industry which would beneficial to his political career (e.g., the first thing that pops would be to accept additional vehicle taxes, which would help Christie carry and receive donations from environmentally conscious voters in a liberal state susceptible to flooding)

(Edit: remove an incorrect assertion. Laws regarding specific individuals are constitutional, just not "bills of attainder")