Makes a lot of sense actually, but I have heard that the tattoos themselves are thought of as "unclean" in a way that the bath cannot clean. Makes me think of, back in the early 90s, we were so afraid to use a drinking fountain, like in public, after someone who was infected with HIV had used it. You know, the big AIDS scare. We didn't know it couldn't be transmitted that way. I always felt like the tattoo taboo in Japan had similar energy.