Newspaper, cardboard, and other smooth papers are dangerous because the chicks can get spraddle leg from walking on them. These things are too smooth and get slippery from poo. Then, the chicks don't have enough traction for walking safely. It's kind of like us trying to walk on ice in smooth bottomed shoes. Our legs go every which way, though some people might manage okay.
Chicks that are placed on things that don't have enough traction can slip around while trying to walk. It can cause permanent damage to their legs. This can be so bad that they cannot walk at all and have to be culled.
Using the shelf liner, paper towels, wood chips, etc. gives some traction for them. They won't slip as easily when they walk.
Note: Some types of shelf liner are REALLY slick. You have to choose carefully. See the picture I linked to. This is the type that has holes all the way through, is soft and a bit squishy, etc. If they spill food on it then it goes through the holes. It doesn't sit on top to get mixed up with poo and make a royal mess, either.
