Why is Hentai Censored in Japan?

I remember reading something about it months ago, but never fully understood.
Who is it censored for? Will there ever be a day where this might change?

Why didn't you order mature anime DVD online to find out from yourself if they are censored or not ? : http://www.animecornerstore.com

Of course if you prefer real AV Japanese girl's action in DVD : http://www.jlist.com