Peter, I worked in Human Resources for many years, and was also an EEO investigator. The fact is, the walls of your workplace are your employer's walls and he has the right to say what may or may not be posted on them. It has nothing to do with the current administration in Washington. Since it seems that your union has come down on the employer's side, I wouldn't be at all surprised if a statement to that effect is someplace in your union contract (possibly with an exception allowing the union itself to post anything it wants that isn't overtly libelous).