A Law Permitting Abortion

In response to Michael S's question below, I want to ask another question.  Is a law permitting abortion a law or the absence of law?  That is, is the failure to prohibit abortion the same as a law that affirmatively ratifies or mandates an unjust act?  Would a society that had no slavery (and had never had any slavery) need a law explicitly prohibiting slavery to avoid injustice?  I know this is not the situation with abortion (or slavery) in our society, but I took Michael to be endorsing a general, conceptual point about laws that fail to prohibit abortion.  So the parallel hypothetical would, I suppose, be a society that had never had an issue with women seeking abortions and therefore never sought to legally prohibit the practice. 

I remember a while back that Steve B. distinguished between morality and law to argue against the mandatory nature of minimum wage laws.  I think his basic distinction was correct, although I disagreed with his characterization of the hierarchy's position on the need for minimum wage laws in light of certain uncontroversial facts about the way the U.S. has organized its economic system.  Would Michael S. reject Steve B.'s minimum wage distinction or is there some bedrock difference between the injustice of abortion and the injustice of paying a substandard wage that justifies a different analysis of the relationship between law and morality for the latter?  I know I've raised questions like this before, but I've never really been able to get the conversation off the ground.