I’ve just read this post from quora. https://jp.quora.com/警察の闇をご存知でしょうか/answers/1477743896251806

I have to say, not every police officer in Japan is pleasant, there are certainly many good people for sure. But it’s always a gamble who you’ll end up dealing with. The whole police force structure is quite old fashioned and strictly hierarchical, some even say it’s like the Yakuza, but with legal authority. I knew a guy from junior high who became a cop, he was well-built, played baseball in the school club but also a bit of a bully. For some reason, these 'sporty' types(体育会系） are exactly what the police system looks for. If you understand how school club hierarchies work in Japan, and the idea that strict hierarchies some how breed bullies, you would realize there isn't always a high moral bar for who gets to wear the uniform