The new robots were 3 laws compliant, but VIKI (also 3 laws compliant) invented law 0 (protect humanity) and imposed it upon the newer robots remotely. They wer

The new robots were 3 laws compliant, but VIKI (also 3 laws compliant) invented law 0 (protect humanity) and imposed it upon the newer robots remotely. They were able to ignore law 1-3 if doing so conflicted with the current plan to save humanity. It was demonstrated (via flashback) that the older robots were capable of allowing a human to die, while ignoring a direct order from a human, and might have been able to rationalize law 0. They weren't remotely linked to VIKI, making them harder to coordinate, so it was easier to remove them from the equation so they didn't get in the way.