They eat a LOT of things that aren't food. Mine ate the peeling paint off the side of my boyfriend's house for a good year or more - they just get weird tastes for non-food items. Also, just because we don't think it's food doesn't mean it has no value. Styrofoam and paint should probably be left off the menu if at all possible, but anything that came out of the ground, flies or crawls is fair game for chickens.
When I give my hens food on paper plates, if I don't get the plate soon enough, they start to eat the plate as well.
It hasn't hurt mine at all and passes right through them. One piece of shredded paper is nothing compared to the amount my hens attempt to eat. They also go for styrofoam and anything else with a bread-like texture.
Recently, my chickens have begun to shred all the newspapers I put underneath on a tray below where they perch. Has worked great for over a year! Each day I roll the newspapers up & especially NOW in the winter, don't have to clean coop as much. But in the last 2 weeks, the coop is a total mess of ripped, torn & eaten newspapers. Think one started, then the others followed (Have 15 of a wide assortment) My guess is because they cant go out in a blizzard of 3 ft. of snow...they are just plain bored
Chickens eat sand and gravel without harm (actually, they need both to survive!) What their digestive systems can't handle will just come out the other end. I'm assuming it's just plain newsprint, without any funny chemicals.