The reason that jail and prison food trays are made so thick is to keep the heat in for the food that is being served.
Many meals are prepared for prisoners and jail inmates at a time so the food can sit on a serving rack for awhile before it's actually served.
The thick food trays help to ensure the food such as meat, potatoes etc stay warm enough to be served so that prisoners and jail inmates don't get cold food.
But I wouldn't care really if inmates had gotten cold meals because prison and jail is not supposed to be a very comforting place and should be made so that people don't want to end up there.