If any real business put up a sign like that, it would be reported in the papers and on the news, causing the proprietor much embarrassment or even legal problems. Once again, a single, isolated incident.
What makes you say that? This is not illegal in Japan. At least most Japanese courts don't seem to think so.

As for the newspapers reporting it, are you kidding? Even with all the work that Debito has been doing about these issues, you don't see the newspapers reporting something like this as news even though there are many examples of these "single, isolated incidents".
http://www.debito.org/roguesgallery.html

I assume you've heard of Debito's Otaro lawsuit. Even though he has been refused entry to an onsen after he became naturalized Japanese, you still don't see any outrage. He makes the news occasionally but only because he is litigating the case.
http://www.debito.org/otarulawsuit.html