Honestly, I think the witnesses' lack of reaction is being blown out of proportion. For one thing, I didn't see anything in that twittered "article" that said a crowd gathered and just watched, yet that's how posters are treating it. People were probably walking by, 2 or three at a time, and saw skin out of the corner of their eye and barely looked. They were behind bushes, after all. And were there really hundreds of witnesses, or hundreds in the park itself? That could easily be misrepresented...
Now, it's been said here that many rape victims wouldn't scream, but how many of those passers-by know that? For that matter, if you walked by and saw that little scene, and there wasn't any sign of struggle, what would you think? She may have been threatened, but how would others know that? They probably just thought it was some couple having kinky sex in the park.
That being said, it's horrible what happened to this woman. The lack of help is tragic, but it doesn't mean we're a cold, uncaring society. We're just stupid, uninformed, and indecisive.