[dragonwriter]

I don't think negotiating ability is the issue, I think that the idea is that women are empirically less inclined to salary negotiation, for whatever reason. Since salary negotiation inclination (or ability, though I don't think that's the underlying reason here) isn't the job, isolating it from pay is a way to pay fairly (not just pay women fairly either, while on average there may be a gender difference here, there are also individual difference within the same gender on this axis).