Because juries in the penalty phase of capital cases must be unanimous, it is particularly important to remove from the jury those members of the venire who will not actually weigh the aggravating and mitigating circumstance but instead will automatically vote against the death penalty no matter what. This is particularly important in states which stupidly do not require the jury to deliberate to unanimity one way or the other but instead allow a single holdout juror to veto the decision of the other eleven.
People often do not state their views straightforwardly. Sometimes they are dishonest, but more often they just haven't thought them all the way through themselves. Determining which jurors are "Witherspoon/Witt" excludable therefore involves some judgment. The trial judge, who sees the venire members live and in full context, is entitled to considerable deference in making this judgment. However, anti-death-penalty judges who are just itching to overturn a death sentence regardless of how richly deserved it may be find jury selection to be a fertile source of excuses to nullify a law they disagree with. To forbid such misuse of the law and limit the lower federal courts' power to overturn state decisions to cases of clear error, Congress enacted the "deference" provision of the Antiterrorism and Effective Death Penalty Act of 1996. Federal district and circuit judges who regard themselves as infinitely superior to state supreme court justices hate this law and regularly ignore its mandate. This is particularly common in the Third, Sixth, and Ninth Circuits. Reversing them has become a significant part of the Supreme Court's workload.
In today's decision in White v. Wheeler, the Court includes the following admonition without dissent:
As a final matter, this Court again advises the Court of Appeals that the provisions of AEDPA apply with full force even when reviewing a conviction and sentence imposing the death penalty.The fact that it is necessary for the high court to so admonish the federal appellate courts is a sad commentary on the state of our judiciary. Judges who cannot or will not decide capital cases fairly should not sit on them. They should be excludable just like the jurors. If they will not recuse themselves, perhaps it is time to establish a challenge for cause. How about a rule that a federal court of appeals judge who is three times reversed by the Supreme Court for failure to obey AEDPA in a capital case will sit on no more capital cases?
Jonathan Adler has this post at the Volokh Conspiracy.