I think there is a fine line there; a company that sells x brand footwear have to tolerate an employee that publicly espouses the virtues of y brand on their own time if they are easily identifiable as an employee of company x?
On something that specific I would say there is a fine line. But if your employer is a Hillary supporter and you post I hate Hillary on FB, etc. and are fired are you being denied your First Amendment right? It is not discrimination in a right too work state, though.