Thinking that someone should have made bad decision and could have made a better one, and blaming them are not the same thing at all.
If a woman was attacked walking home from a nightclub at 2am, I would not say "it is your own fault".. but I would think that if only she had stayed with her friends and not gone off on her own, that it most probably wouldnt have happened, and that she made a very big mistake by not putting more importance on her safety. I am fairly sure that the woman would be wishing she had done so too.
Yes, she could have walked off with a known male friend who could have raped her. Yes she could have gone home and her boyfriend raped her. Those are not the type of cases I am talking about though. Those are also not the cases that posters and advice to women are about. The posters and advice are more aimed at reminding women to be aware of the possible danger from strangers. And as I have said, just because it is only a small fraction of the statistics, it does still happen. Maybe the reason it is a small fraction is because more and more women do listen to the advice and do avoid being out on their own if it is possible to do so, and if the number of stranger rapes could be reduced even more, then surely that is a good thing.