A little more technical answer. WiFi is legal in Japan, but there are few small differences compared to US or rest of the world.

The 802.11 standard specifies 14 channels. Japan permits all 14 channels to be used. US only uses channel 1 through 11, and the rest of the world allows channels 1 through 13.

What does this mean? If you take an US-spec adapter to use in Japan, you may not be able to see the wireless access points on channel 12-14. There may be a country setting in the device driver that will enable those channels. Likewise for the access point, it may not be compliant to the local regulations until you change the country setting in the configuration.