Honest question: Which law or code does it violate?

It's not like he's refusing to hire people of color or Vietnam veterans or even transsexuals. (Edit: I realize it might sound strange, but there's a legal basis behind that ordering it's not just my preferences :-)

He's refusing to hire certain resume' histories arguing that it is professionally relevant.

First of all this is non-merit discrimination; which should bar yammer from doing business with government, and contractors. Check out their fron-page, the top of their customer list is Pitney-Bowes.

Second, he is creating a hostile work environment for his own employees who may feel differently about the case, or who have come from companies that collapsed as he is expecting Yahoo would.

I think he's saying that it is an issue of merit, in the sense that he considers it a negative indication of the judgment of an applicant.

Hostile work environment? Meh. Sure maybe somebody could bring a suit, maybe they could even have a chance at winning if he was beating people on the head with it every day. But I doubt many startup execs would worry about it to the point they would let it censor their behavior.