Japan's laws on sexual content and decency. Pornographic content isn't actually allowed, but the work around for that is censorship which upholds "modesty, shame, and sexual morality." This allows them to produce and distributed adult content while retaining the law. While they could just remove the law entirely, publicly that would look like they're giving in to degeneracy, giving the current government a bad image. Being an Eastern nation the general belief is to uphold decency and self-restraint. Nobody wants to be the one to pull the trigger and omit the censorship rule, so the work around is to interpret the law in a way that allows for porn through self-imposed censorship. The rule still works and so there's no need for them to change it. While I'm sure most Japanese would prefer uncensored material, the issue is minor enough to them that they probably wouldn't care to lobby against it.