Why did Miley Cyrus do this?
Who knows?! She might have acted like this just to shock the society, or to attract attention upon herself and to gain benefits from it. Or may be she wanted to copy some other pop stars from the past, and to repeat their success by showing how scandalous she can be. Whatever her reason was, it's not right to act like a cheap naughty stripper in front of so many teenagers and under aged children.
You know how Madonna became famous, right? It wasn't only her talent that presented her to the world, but her scandalising personality and nudity. It is lust, disgust, horror and other uncensored and tabooed things that attract attention. "Sex sells", as someone once said. But it shouldn't be shown in front of young grow ups.
The whole world watched Miley Cyrus shaking her not-very-well-shaped behind, just in lingerie, and rubbing it into a man's crotch with her tongue stuck out. Moreover, she used a fake finger and made a clear demonstration of how it can be used for sexual satisfaction. The crowd got wild and everyone spotted her, but now everyone is judging her behaviour. May be that's what she wanted - to be spotted and branded as the new bad girl of pop music. If that was the idea, she made it, because everyone has been talking about her for the last two days. It was a very cheap demonstration of inappropriate behaviour, though.
You already got my idea, so I won't go any further into judging Miley Cyrus. I just want to say that with this performance she became part of my list with Celebrities Who Don't Deserve Our Attention.