I agree with most of what you're saying, but I also think you're kind-of side stepping the issue that the paper sought to quantify.
Yes its not just a problem for women, but there's a lot more pressure on women to avoid confrontational negotiation[1] because they'll be branded much more negatively (ie "Bitchiness" or some other term), and if they are, then they risk being taken less seriously.
Men don't have the same problem, as culturally strong/pushy personalities (that might fare better in negotiation or negotiate more) are seen as less of a cultural negative. It's something explicitly expected of one gender and seen as more repulsive in the other, and that becomes a big deal for things like salary negotiation.
[1] See Women Don't Ask: The High Cost of Avoiding Negotiation - http://www.amazon.com/dp/0553383876/
For what it's worth, there's what looks to be a strong rebuttal of most of the things this book says here:
http://fdiv.net/2012/01/20/pseudo-science-and-pseudo-feminis...
(I'd tend to think that your point stands anyway as being obviously true, but after reading the rebuttal again along with the link to this article I'm not sure anymore.)