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?

http://mdn.mainichi.jp/culture/waiwai/archive/news/2007/09/20070911p2g00m0dm008000c.html
Because it's the law, a law still in force, in the realm of photo-porn anyway. Not sure how compliant drawings need to be. Maybe people just got used to it, so it's included as a style thing now.