Is it possible that in the New Era, Kenshi is basically an American citizen growing up?
Firstly, in a report from 1985, Bill Phillips of the Los Angeles Police Department's organized crime intelligence division, has confirmed that there are Yakuza affiliates around LA, but they are not what you'd call upper management people, rather, they are scouts.
Secondly, in California, the yakuza have formed alliances with Chinese triads, Vietnamese gangs, and local Korean gangs.
Thirdly, in order to join the FBI, you are required to be a US Citizen, have no criminal charges, and pass lots of tests, like sobriety, drugs, etc. I doubted that when Jax asked Kenshi to join, Jax would know Kenshi's Yakuza past and if Kenshi isn't an American citizen, he would have a lot of trouble to go through the process of applying and in a nutshell, he wouldn't get the job straight away.