Tesla’s Fremont location in California has been required to put up signs explicitly stating that workers can unionize, following anti-union efforts. An NLRB found the firm guilty of discouraging employees from wearing union-related apparel. Additionally, Tesla has a history of tension surrounding the issue of unionization. In order to ensure that employees are aware of their right to unionize, the board decided to order the firm to put up signs.
See "Tesla was required by the National Labor Relations Board to post signs at its Fremont factory saying its workers can unionize following accusations that it tried to discourage unionization efforts (TSLA)", Mark Matousek, The Greenwich Time, September 20, 2019