To be honest, i dont see anything wrong in the way the guy was treated, he is an attention seeker and small-minded too, despite his intresting writing style, i think he has some confidence in himself issues..
Some things i noticed with his story, although he claims he is trying to no persuade anyone of his innocence, he seems to be going to great lengths to put an image of being overwhelmed by some violent yakuza thug in a mafia style car full of violent hoodlums, which when you look at the evidence, doesn't seem to be the case;
a. "reasoning had failed and I was standing in front of a car full of violent hoodlums"; He describes the car passengers as "violent hoodlums", but there is no show of violence on their part neither any interaction at all.
"I explained repeatedly and to no effect that I was waiting by the side of the road for his car to pass." To tell somone somthing over and over repeatedly, at least in my books, is not really what i call "reasoning"- for somone who has lived in japan for many years and has good social relatiionships, i think the wiser thing he could have done is to appologise and walk away- drawing your attention to yourself, especially if you honestly fear for your safety,, is not a wise idea.
b. "Panicked, I pushed him through the window as a warning to desist and turned to go. The driver jumped out of his car, escalating the verbal assault and now threatening to sue me."
HE was the violent one, HE attacked a person he did not know, he pushed them through their car window. At the least, technically he is the one in the wrong, although he may have been verbally provoked, up until this point there had been no act of physical agression and thus he should not have pushed the guy- it was not an act of self defence. He should face up to this fact and admit it.
Neighbourhood gossip is irrevelevant to his situation, what done is done and it does no matter if the car guy had successfully sued people before as it did not in any way effect his meeting with the "yakuza man" or the consequences of the meeting- it does not make him nor the other man any more innocent or guilty of their actions.
The fact he is calling the man member of the yakuza is a complete assumption with no evidence to back it up, and yet again this does not change anything about what happened- this man does go to great efforts to make the people reading his story to really dislike the man he pushed.
The fact that the interogation room was dirty is just unfortunate, but should not have effected the truth of his situation- when it comes down to it, it is unlikely the room would have been completely unsuitable for sitting and talking in for a couple of hours.
The interrogation methods are very standard, they are used pretty much world wide and he is no exception- given that he was the agressor in the whole ordeal, its reasonable the police make positively sure that there was nothing more to the story.
I can see nothing wrong with the way the police officer treats him at all, he was treated very fairly IMO.
The way i see it, i can see no discrimination directed towards him simply because he is foreign, what he experienced in japan as far as the interogation was concerned would have been done about in exactly the same way over here in England or America as far as i know.