The Constitution not only permits such questioning, it arguably requires it. Although the Constitution makes judges independent after appointment, it sets up an explicitly political appointment process before a judge is approved. Why on Earth would determining a nominee's approach to interpreting the Constitution be thought to be out of bounds, before giving her a lifetime appointment to do exactly that?
Is there any line of inquiry that the Constitution does not permit? Yes. It would be improper to try to exact a pledge as to how a nominee will rule in future cases. As long as the inquiry stops short of that, it does not violate the Constitution's protection of judicial independence, nor does it violate judicial ethics. Parties before the courts are entitled to judges who will consider their cases without bias. But they are not entitled to judges who have no views of the law. An open mind is one thing; an empty head is another.
We would go one step further. The most useful way of discovering a nominee's views is through "litmus tests." One question would yield the maximum information about a nominee's judicial philosophy (without requiring a commitment as to any future ruling): "What do you think of Roe vs. Wade"? The answer could explain her theory of constitutional interpretation, her views on the judicial invention of rights not set forth in the Constitution, her views on when courts should follow precedent, and her views about the judiciary's role in our constitutional system.
Now, where were these notable conservatives when Democrat senators' questioning of Roberts on Roe was considers unacceptable and unethical?
Oh, here is Yoo on Roberts' senate hearings:
A member of the Senate can choose to vote for or against a nominee for whatever reason he or she chooses, and can ask any question they want.
At the same time, nominees have a duty not to judge cases before they are argued and submitted to them for decision. We would think it wrong, for example, for a nominee to announce during confirmation hearings that he or she will always vote for or against the police in a search or seizure case. That limits their ability to answer.
Q: Senator Specter said that he intends to ask Roberts if he believes that there's a Constitutional right to privacy. If Roberts says no, is this cause for concern? In other words, do most judges believe that such a right is implicit in the Constitution, or would this make Roberts something of outlier?
John Yoo: That would be a fair question to put to Roberts, and it has been put to previous nominees. Given Roberts' respect for precedent, he will probably answer the question yes. I would be shocked if he said no. At worst, he would say that he would not answer the question so as not to prejudge any cases coming before the Court now about the right to privacy.
Q: ...what extent do you believe a potential Supreme Court candidate's views on contentious issues (abortion, affirmative action, etc.) should be considered as criteria? ... I wonder if, in the hearings, the Senate should consider not just what Roberts' views are, but his logic at arriving at those views. What are your thoughts on this?
John Yoo: ... I think that President's should consider judicial philosophy when they appoint Supreme Court justices. ... I think that Presidents would not do their duty if they appointed judges they thought would undermine or disregard the Constitution. In order to make sure that doesn't happen, Presidents must inquire at some level about what a potential nominee thinks of the Constitution.
The important thing, however, is to recognize that there are areas where the Constitution is ambiguous, or where reasonable people can differ. Both Presidents and Senators should realize that there are areas where they can accept someone who has a different view than their own.
Note that Yoo did not answer the question. First, he talked about the President, not the senate, as the questioner asked, and second, he does not say weather the senate should consider Roberts' view on controversial issues. And certainly did not express the views of the later L.A. Times op-ed.
Conclusion: Yoo is a torture enabling hypocrite.