That's interesting, SJ. I know police in Shinjuku are fairly active in questioning foreigners and lone young Japanese guys but I assumed they mostly took the initiative.
Another point I don't quite understand about this fiasco is the length of his incarceration. I read speculation that he refused to sign a gomen nasai letter and presumably the police didn't want to let him go free without something to show for their effort. I'm wondering if it took ten days for him to work out it was OK to sign or whether the prosecutor eventually figured there was no mileage in keeping him locked up for a misdemeanour and just let him go.
I think if I'd held out against signing anything on principle, I'd be pretty furious and want to shout about what happened from the rooftops. This chap, however, doesn't want to be named. Perhaps he and his son are pursuing some private action but there is no mention of that.
All in all, I would say the police need to review this policy of randomly questioning people who approach them for help, if indeed that's what happened. On top of that, this particular case ought to give police and prosecutors pause for thought about proportionality because this has caused a lot of unnecessary grief regardless of the letter of the law.
I'm not sure that foreigners can make a big deal about how they don't know what the law allows. After all, it was only relatively recently that Debito was on the case of a knife shop in Akihabara which didn't want to sell to tourists so we can't have it both ways. There might be a case for suggesting that signs go up at major ports of entry to warn that carrying a pocket knife can be deemed illegal.