And that is not what's happened here. AT ALL.
To continue your idea; let's say that I was the mayor of a city. After the election it comes out that they were investigating my campaign.
Then I tell the police commissioner that detective needs to be investigated.
(side note, the department of justice is supposed to be independent and ignore things like this from the President, but they didn't so oh well).
The commissioner says "Yeah, let's do that."
The detective who ran the case isn't put in charge. Internal Affairs runs the investigation.
Which is exactly what happened here.
An internal group whose only job is to investigate the department investigated the department.
That you can't understand that at this point shows you're willful ignorance.