They are bored because they routinely ignore cases of domestic abuse, sexual assault and harassment, murder, they have corrupt agreements with the yakuza, and on and on it goes.
Well, many people would take the first case (no crime, and the police trying to find the smallest bs as crime to pass their time), over a country with lots of crime.
(Sure, there's also the perfect case, no crime, and the police sitting idly, but in the real world we rarely have the "perfect" case, and "no crime/police fishing for people stealing a beer case left as a thief trap" is close enough to perfect).
Injustice anywhere is a threat to justice everywhere, and this is a clear example of injustice. It's a slippery slope to effectively say, "we sometimes punish the wrong people, but in return we get a very safe society", because how can you feel safe in a society that may, at any time, randomly select you for punishment? It might not be "crime" per se, but when you're locked up, does it really matter if it's all by the book? Doesn't that mean the book is bad?