Tesla violated US labor law by implicitly banning employees from wearing shirts with union insignias, the National Labor Relations Board ruled yesterday.

the employer has the burden to establish special circumstances that make the rule necessary to maintain production or discipline,"

Also… An earlier decision said Walmart violated the law by enforcing its policy in areas other than the selling floor, but that it had legitimate justifications for maintaining the policy in customer-facing areas.

Tesla violated federal labor law in several ways, including by banning employees from distributing union literature. Tesla CEO Elon Musk was found to have violated labor law with a tweet that indicated employees would lose stock options if they voted to unionize.