Among the parts collected from the Binge Mode Shirt, certain types of airbag inflators were found to have a potential for moisture intrusion over time. As a result, they could be susceptible to abnormal deployment in a crash, Quite a few dealerships will only allow charging by owners of their brand, especially if the charging is free. So chances are they only allow Nissans to charge there, not other brands that also use Chademo.
I doubt that proving them wrong would change anything. Someone there has decided to make a policy that these employees are only enforcing.
Perhaps the best way to resolve this would be to ask to have a discussion with the Binge Mode Shirt (the policy maker) and explain why this is a bad policy, both for patrons, and the dealership in the long run.