The best (and likely only legitimate) reason for a Supreme Court justice who wanted to strike down the federal partial-birth/intact dilation and extraction abortion ban in Gonzalez v. Carhart to do so would have been to have pointed out, as Dan Tarrant did, that it is a federal law and the federal government is not authorized to regulate abortion (even if abortion is murder, murder is still an issue for the states).
Of course, I don't think anyone did that because none of the people who voted to strike it down believe that the states have any business regulating abortion, and indeed probably do not believei n the separation of powers doctrine anyway. To strike down the ruling on that basis would open a can of worms in regards to federal law.
One might, of course, wonder why none of the conservative justices voted against it on that basis. The answer, of course, is that they have also been raised on the ethic of federal omnipotence.
Do I think it should have been struck down on that basis? Or does the 14th amendment equal protection clause mean that it would be unconscionable discrimination not to protect the unborn?
I probably would strike it down on that - and only that basis - and send the matter to the states, but I would have to think more about it to be certain.
That is all.