The carnist claims that since X isn't a necessary ingredient to making product Y it isn't a moral consideration. E.g. Shredding baby chicks alive isn't a necessary component of making eggs, and therefore the fact that chicks are shredded is not of moral consideration when buying eggs.
It does not matter that stealing a car isn't a necessary component of driving a car, stealing a car is wrong. We don't judge an action based on whether or not it could be moral, so that can't be your defense if you're driving a car you stole.