Because those people are compatriots of those scumbags.
If I may - the concern about torture is not about the murderous scumbags who are subjected to it. It is about us.
If people crow about “the rule of law” and then want to disregard the rule of law whenever a ‘necessity’ can be articulated, there is no rule of law. Without getting into hair-splitting over the techniques involved, waterboarding is illegal. We have executed people for performing it. I don’t really think any conservative wants to go down the “okay for me but not for thee” road.
If we are going to take the position that this is ‘okay,’ then we should withdraw from the Convention Against Torture and the Geneva Convention and not make further efforts to enforce or conform to any recognized norms on prisoner or detainee treatment. “Might makes right” can seem pretty attractive when one has the might, but there isn’t anything “conservative” about that approach.