I don't know how it works in China but where I am a person's criminal record is not public but not exactly private either. In the sense that an employer can ask for your criminal record and you have the choice giving a printout of it or not having your job. Making it kind of hard to see how the knowledge of a criminal record could be used to blackmail someeone.

As for "data brokers. Advertisement, financial credibility, trustworthines of buisness partners etc.". Maybe. But these companies would turn themselves into criminals by using or purchasing this information.

It is likely, that this DB contains more information, than what a formal printout gives.

"But these companies would turn themselves into criminals by using or purchasing this information."

Which is why they probably would not deal with the information gathering directly, but use a service of a data analyst company. When they do something illegal, nobody who contracted then did ever know anything. I think this game is played in china as well.

rest assured that intelligence agencies have means of accessing police records in other nations.

this data is only interesting to the low end of data brokers, advertisers and other scammers, hence the rather low price.