what you heard might have been confused with cordless telephones? those type of phones ARE illegal in Japan because they use some of the frequencies that Japanese emergency communications use. So we arent allowed to use non-Japanese manufactured cordless phones here in Japan.

I havent heard anything about wireless routers other than the ISPs say you cant use ANY kind of router because they want you to pay for 4 accounts if you have 4 users. But no way they can tell if you have one and I am not paying for 4 connections for all 4 people in my house, paying for one is enough, so wireless router it is.