You're side is devoid of common sense. Can a public employee, any public employee (Professors are not yet a protected group) announce that they will withold personal recommendation for advancement based on race, religion or gender?
As I understand the laws of this country, yes.
As long as it is a "personal" reccommendation.
I'm in IT. I sometimes write letters for folks whom I've worked with, to help them get jobs.
I could decide to only write such a letter to a white, stacked, beautiful young woman, if I so chose. And I would have broken no law.