That's just incredibly naive. If they were serious about corruption, they'd have to dismiss the entire police farce (including immigration) and start over. Even their internal processes are hopelessly corrupt, starting from passing the entrance exam. All that is well-documented, of course, so you're either having a laugh or being delusional.
So who gets caught and is punished for "being corrupt"? Officials who fail to pass the appropriate share of the loot up the chain. Officials who cross their superiors in other ways. Officials who have the wrong political affiliation. In all those cases, having some "corrupt officials" as fall guys to make an example of is perfect both with regards to the effect on the gullible public and as a deterrent for other officials. Nothing more, nothing less.