How the fukk was it legal in the first place?
The Japanese embrace sexual fetishes more than other societies, which is why stuff that seems ridiculous to us is so common place there. The reason possession was legal was because the Japanese objected to penalizing people for having certain sexual desires, regardless of their questionable nature. The reason they're finally going to penalize it now is because they could no longer act like the consumer demand (the legal aspect) didn't promote the production/distribution of child porn (the illegal aspect).
This is also why child porn in hentai is still allowed, because it cancels out the aspect of child pornography the Japanese object against (the abuse of real children). Basically, Japan prefers their pedophiles jerking off to child porn hentai at home rather than have them out on the streets prowling for real children, which is a philosophy that in essence makes sense. It's just that up until now the laws themselves made absolutely no sense.