Longing for the old days, I'll wager, when a gaijin was a gaijin and you could spot them a mile away. Accept, you know it never was like that, because the largest gaijin-card-baring ethnic group in the country is Korean, and they pass easier than other ethnic groups. So they pick up this woman and shake her down like she was an over-stayer, and she turned out to be a citizen.
I predict the police will say that this kind of error is bound to happen, but it is worth it to insure the public safety. Safety from what? The statistics don't support nationalistic theories that foreigners commit more crimes.
Police left red-faced after arresting Japanese woman they thought was a foreigner - MSN-Mainichi Daily News