Law Professor David Yamada reveals that "in many instances the law would allow a manager to terminate a white supremacist for participating" in a public rally, "but there are potential exceptions." 

Yamada gives an overview of the relevant legal rules in the blog post “Can an Employer Fire a Publicly Avowed White Supremacist?” on his New Workplace Institute Blog.

Yamada is is an internationally recognized authority on the legal and organizational implications of workplace bullying.

Full blog post