The employees at Tesla were told not to wear any clothing that is pro-union. As a result, the Tesla company broke the labor laws since it required the employees to wear the company uniform. If we look at Tesla's opinion, the reason for mandating the production workers to wear the company-provided uniform was for quality control and safety.
According to Tesla, it operates better for the employees in a setting of non-labor nature. Similarly, Elon Musk (CEO of Tesla) has been an open critic of the labor unions. Recently, he even encouraged the labor union groups to hold an election at the company facilities.
According to the sources, Tesla has broken the labor laws by not allowing the employees to wear any clothing that is pro-union. The agency responsible for looking at the matter also reported against Tesla by 3-2. It was 1935's National Labor Relations act for which Tesla was found to be in violation.
As for Tesla, the dress code measures were introduced to prevent any damage to the cars arising from clothing. On top of that, the company uniform also promoted safety and quality control during the production process.
Tesla also made it clear that it didn't stop any employees from showing union insignia on top of the company uniform.
The root of this matter can be traced back to the 2018 testimony of Tesla employees - It was revealed that the employees were told that they couldn't wear any shirts that support the UAW.
Now that it was brought to the attention of NLRB, Tesla will have to make several changes to its dress code policy.
As long as the union short is similar to the Black apparel provided by Tesla, the employees should be allowed to do it!
On the surface, it may look like a simple matter, but it will empower Tesla employees. In fact, it may even lead to the establishment of unions in the Tesla company.