Not a lawyer but consider the following:Imagine thinking your shit island nation holds any kind of legal leverage overseas
ROR
RMAO EVEN
They are playing with fire by making this public. Sure, this notice alone could scare some operators into pulling the plug before they start getting wrapped up in the nonsense that is Japanese defamation law. However, this could embolden the biggest sites to simply host out of and incorporate (or re-incorporate) in the USA.
They can probably always fuck with Japanese nationals if they know who posters are, regardless of where the site is located. Ultimately, a US site would be under no obligation to give up any user information, like posts made from Japanese IPs.