Hypothetically speaking, by that logic a black rapper who talks about being a gangster would be reason enough to assume that other black people around him are also gangsters. So if an employee said of a group of black rappers "the gangsters are out today" under your logic it would be wrong to fire that person, even though they couldn't possibly know what that person was, is, or wants to be.

You're assuming guilt by association, and probably falling to Godwin's law. There's a difference between what someone may be, and what you may say in the workplace. The fact that the person who said is is Jewish means nothing, it's only a distraction from that an employee referred to a group of people as Nazi's and the employer took a offense and..the employee is looking for a new job.

You can't fire someone for behavior and excuse it in the other simply because you agree with their politics.