That would be discrimination, and I imagine the company would fire the employee in that case.
The difference between that scenario and what’s happening is that the employee is also an agent of a union, not delivering mail to Tesla is part of an ongoing strike, and the right of unions to strike is protected. If a union or individual employee decided not to deliver mail based on ideological differences, not as part of a strike, that would not be protected. If the strike ends and they continue to not deliver mail to Tesla, that would also not be protected.
This may be the most apt application of this meme I have ever seen, lol