What proof do you have that this is what happens? Please provide it. Thank you.
If the newpaper runs it, which there is a good chance if you live in a smaller town, the department will feel compelled to at least respond to the allegations.
About it going to the shredder, working for 2.5 years in the Hall County justice system pretty much taught me how complaints were handled in their office at least. It had to be a very strongly supported allegation to get any kind of attention and complaints about fishing were not addressed. Granted the idea that they go to the shredder is hyperbolic, but most complaints are filed away and simply ignored unless the same officer gets into other kinds of trouble they can bring it up against him. I got fished out one time because I took a detour through the wrong side of town, they claimed I was swerving, I asked an officer at work about it and he pretty much told me it was fishing and there wasn't a whole lot to do about it but file a complaint which wouldn't really make much of a wave in the ocean so to speak.
I don't think anyone could provide "proof" that this happens unless they had a personal experience in which they tested both methods of filing a complaint, maybe someone who has pursued filing complaints or writing letters will enlighten us with a more objective observation.