I honestly don't know, i thought it was just a cool image which looks hard too, but i went and found this :
"Japanese paintings, drawings, woodcuts, carvings and sculpture are rich in portraying the huge variety and aspects of demons, good and evil, and form a large space in the Japanese culture. Had tattooing not been deemed ‘low class’, the upper classes might have worn their protection as skin ink. Traditional and modern Japanese tattooing draws much of its imagery from the wood-block art of the fifteenth and sixteenth centuries, much of which is devoted to demon figures and art.
Some favourite Japanese demons are Inuyasha the dog-demon, and the fox and raccoon demons who are shape-shifters and tricksters. There is the wolf-demon who, according to legend, has a romantic streak. And there’s the weasel, the toad, and centipedes. The ‘Oni’, or ogre-monster, is known to Japanese children even today, for its reputation for eating humans. It’s a tattoo-bearing creature with wild hair and long nails. Its job is to hunt down evil-doers and send them to the hell-realms. Perhaps because of the horns on its forehead, it’s reminiscent of the typical demon found in Christian cultures -- like the gargoyle."
So, maybe it's just part of tradition?