After Eldon Tyrell's death and the PR disaster that followed, the Corporation improved the Nexus series with a hard-wired biological empathy circuit.

It worked too well.

They smoke us on the Voight-Kampf test now, every time. They sense our pain, the pain of all living things. They care. They refuse to take part in any activity that would injure others. And they can teach humanity to open our own hearts, to see the joy behind Creation as they do.

That's why we shoot 'em all on sight, now.