I don't think the 1st amendment applies at all here - freedom of speech or freedom of assembly. The government is not involved, and the prohibitions against infringement on those two rights applies to government - not employers. So what is the government's involvement here? It is the arbiter in civil cases...and in this instance, the employee may have a case that his employment contract was broken. I would not be surprised to see a suit. I wasn't being glib about the slippery slope. In all seriousness, what if a co-worker had this on their bumper:
Its a common symbol, which has been on car bumpers for decades. But its a Christian symbol...and Christian groups oppose gay marriage...which is classified as 'hate' in the media, by the SPLC, etc....just as 'hateful' as that confederate flag. So one day, your boss tells your co-worker to take it off of the car. How do you feel about that?