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?

You got to love them with their cheerleaders ' dancing and their new uniforms : http://www.nba.com/mavericks/dallas-maverick-dancers

And I feel that among all the dancers, Nazlie is the best : http://www.nba.com/mavericks/dmd-nazlie