Since when does calling for murder and asassination fall under the protection of the First Amendment? The professor is certainly entitled to his opinion, but he is not entitled to advocate murder. Were he a conservative, this would be hate speech and there would be no "investigation" or sops to the First Ammendment. A Conservative would have been fired within seconds of sending the email.
Yeah, you would think this is prosecutable. Especially if someone followed their advice.....inciting to violence.
"...yet I must maintain that the college be a place for free expression, ideas, and open debate."
OK--Let's test that assumption. How about a symposium on the positive effects of slavery, or a discussion of why women should be stripped of the right to vote because they rely more on emotion than facts.