I always assumed that this practice was widespread. That is just an extension of the traditional practice of "tricoche" (police officers performing investigations for private customers).
Despite what you seem to think, police are not more righteous or just than any other person, as we've seen through the constant abuse, corruption, and violence perpetrated by LEO and intelligence agencies. In fact, due to the kind of people drawn to the positions and the screening to filter out people who are too intelligent to blindly follow the state, I would argue that "these people are in fact MORE likely to fall for corruption or "sell their soul just for money" as you say
Corruption in the Baltimore Police Department.
The Seven Five (a documentary about NYPD corruption.)
Operation Countryman - https://www.theguardian.com/uk-news/2018/aug/19/files-shed-l...
Corruption in France: http://www.reuters.com/article/2012/10/05/us-france-police-c...
Corruption in the DEA: https://www.google.com/amp/s/arstechnica.com/tech-policy/201...
Sounds right in line with my understanding of police
>I would expect to see many smart people who have a true vocation for the service
Too bad we end up with, partially due to agencies screening for the particular personality types they can use and control, power tripping sycophants with no critical thinking abilities who will blindly follow the state while simultaneously breaking the rules they're meant to enforce for personal gain
>but I would not expect to see greedy treators. Maybe, I have a crazy theory
It sounds like you have an incredibly idealistic view of police officers due to your father, which is understandable.
>What if this guy was selling this data to try to catch big fishes ?
Highly, highly unlikely
>A person frustrated by the inertia or other ineffectivity of the service and experiencing other difficulties in their life may fall into this madness.
This sounds like a poor excuse to defend crooked LEO
> I think they have regular psychological assessment because this job is very hard.
They have psychological assessments so the government can ensure they will follow orders, have no dissenting opinions, and will blindly defend other LEO/intelligence officers even when they are in the wrong
That's why intelligence personel are themselves usually subject to surveillance and intelligence.
I am just an imaginative layman. My source of knowledge about police is my father (RIP) who was a police officer. He had friends who worked in dgsi.