Keith Humphreys makes a distinction:
[W]hat we do with scientific evidence is always a political and moral judgment. We don’t provide health care to the sick because the evidence forces us to. We provide health care to the sick because such activity is in keeping with our values. Likewise, we might choose to morally oppose certain policies (e.g., capital punishment) even if there is solid scientific evidence of benefit (e.g., if it is ever shown conclusively that capital punishment reduces crime). It would be dishonest to hide behind the evidence and say, for example, that science made us put a helpless human being to death; that moral judgment falls on all of our heads.
The effect of the death penalty on crime may, for example, be unknowable. But I'm against it anyway, on moral grounds.