As matter of fact, the first amendment doesn’t protect their employment. Tenure only means that the professor has a right to due process. A tenured professor may be fired for being incompetent or behaving unprofessionally, if the university is shutting down their program/department, if the university is in financial trouble, etc. At most universities a tenured faculty member can be terminated for violating non-discrimination policies of the university.
However, no university can fire a faculty member because of things which they say as private citizens, outside of work, without mentioning where they work. So if he had signed his screeds or articles that he was a professor at IU on any of his racist rants, he’d be in trouble. if he said anything racist in class, he could be in trouble. Unfortunately, Rasmusen has been fairly meticulous about avoiding doing anything which violates his university’s policies.