 Price 1996 (chapter 5) and Sklar 1993 hold that this nomic restriction is unwarranted or unexplanatory. There is the sense that the causal asymmetry should come out of more basic physics, not be baked into this physics. Horwich 1987 is an example of someone trying to derive what he calls the fork asymmetry, which is similar to the Law of Conditional Independence, from more basic assumptions. 