This is a staple of japanese law. They make a paragraph so poorly worded that it applies to everything and nothing at the same time.
There was a law suggested by Ishihara Shintaro back when he was still governor of Tokyo, that was originally meant to ban lolicon-manga, however it was written, it would ban "indecent content inapproriate for young readers", thereforeso it wouldn't have applied to adult entertainment in the slightest - instead it could be applied to everything from Anpanman to Berserk.
The silly mosaic censorship of their pornos is the same. Porn is forbidden, but with the censorship, it's not considered porn.
Bonuspoints: Back when the law was introduced, the ones doing the censorship for print media manually by putting lines over the naughty bits, were mainly part-timers, so highschoolers and mothers working while their kids are in school.
Japanese laws are fucking exceptional.