I doubt there's much to explain. I assume Will's just deciding, for whatever reason, that not being discriminated against is a basic right, but choosing who you hire and why is not. Everything you say in response is completely correct, though. People should have the right to be wrong about race or sex or religion if they want, and for their bigotry they will lag behind firms that hire based on merit.
That said, I think he's not so much making an argument as just listing something which sounds bad, politically, which he has often regarded as the same thing. I've had many discussions with Will where he appeared entirely uninterested in whether or not something was actually defensible on principled or policy grounds, but cared only about is political viability at the moment, or its susceptibility to demagoguery. So you might be wasting your time in trying to explain why something is technically wrong; it's not clear he cares.
That said, I think he's not so much making an argument as just listing something which sounds bad, politically, which he has often regarded as the same thing. I've had many discussions with Will where he appeared entirely uninterested in whether or not something was actually defensible on principled or policy grounds, but cared only about is political viability at the moment, or its susceptibility to demagoguery. So you might be wasting your time in trying to explain why something is technically wrong; it's not clear he cares.