It's to challenge the assumption that a woman's appearance or behavior has any bearing on whether or not she "deserved" to get raped. A Canadian cop said something like, "If y'alls women don't want to get raped you should stop dressing like sluts." Truly we live in enlightened times where challenging such an assumption necessitates protests and sloganeering.
As for reclamation, I don't think the use of slut by white feminists is analogous to the project to reclaim queer, which has both academic and taxonomical functions currently.
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:expert
Of course people are going to focus a lot of their activism and advocacy in communities and institutions where they already have a stake. It's pragmatic as well as personal. Your college has a really shitty way of dealing with rape allegations? There's a shot you can actually change that. Girls born in Taliban-controlled areas of Afghanistan are going to be horribly oppressed? Not only are the odds you can help pretty infinitesimal, but that "help" would probably come in the form of vast amounts of aircraft ordnance, which has a rather sketchy record for improving people's lives*.
nacho, can't remember you posting terribly much about the plight of women in the third world. Maybe that's from me being unobservant or from being off the forum for a while. But if you are bringing these issues up primarily or exclusively to tell American women they have it good enough that they should shut up... that wouldn't be a great look.
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Which isn't to say American and other first-world feminists don't bother with those issues at all. There was a kerfuffle when Code Pink took a position against withdrawal from Afghanistan, on the grounds that peacekeeping troops were important for protecting the women and girls there, for example. Plus all sorts of feminist organizations making statements and releasing papers about global women's issues, which usually get noted on A17, or wherever the paper runs its Also Around The World bullet points.