Women don't earn about 18% less than men (OECD median, 2008)[0] because a small minority of employers pay women almost nothing, but because the vast majority of employers pay women (or blacks, or handicapped people) slightly less. Just like laws that prohibit killing don't target psychopaths, laws that prohibit discrimination don't target the KKK; they (should) aim to change the societal norms that your average Joe McAverage conforms to. It's the distinction between institutionalized discrimination and the more intimate personal type.
_____
[0] The exact number is not really what's important here, but http://www.oecd.org/dataoecd/54/57/40846335.pdf