a case of pattern recognition in the bathroom

Another pattern recognition example. A while ago, I had read on "How Magazine"'s blog about these design toilet papers, from Portuguese company Renova, and I filed it certain that I'd use it for something.
Now, one of my favorite bloggers, Lisa Katayama, who writes the interesting and incredibly funny Tokyo Mango, came up with these examples of printed toilet papers. She says that these printed toilet papers have existed in Japan for a very long time.
I can perfectly understand "doing your thing" with a black toilet paper, with "skulls", or with the "inflation dollar bill", but with the "Happy Birthday" one, I guess I totally missed the cultural manifestation here.