Again, no one thinks the Slayer posters and cow skull represented Satanistic things.
Just wondering what Janis saw on the bookshelf, that's all. It may have been a lead of some kind. I would think even in the bible belt that the Christian Barney Fife Deputies are smart enough to discern the difference between cow skulls/metal posters and ritualistic things.
I am sure they were just confused by the mausoleum break-in, skulls in tree thing combined with the cow skull in Mike's sister's/Mike/Joe's apartment. That is a pretty dark and odd thing. It's not something that happened much before or since in Springfield. So the frame of reference was new to the police.